Text-book of hygiene; a comprehensive treatise on the principles and practice of preventive medicine from an American stand-point

Front matter

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Copyright N (

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Preface To The Fourth Edition

The advances made in hygiene and sanitary science, more especially in the field of causation and prevention of infectious diseases, made it necessary to subject this well-known and popular text-book to a thorough revision. Several of the chapters treating of subjects in which the discoveries were more recent were entirely rewritten, while others were brought up-to-date by including such matter as appeared essential in the light of recent advances.

The aim of the editor has been to preserve in the book the qualities which made it one of the most popular text-books on hygiene in the English language.

As no one man can be a specialist in all branches of hygiene and sanitation, it was necessary to secure the assistance of other men. This had been done by the author in the former editions of this book, and the plan has been followed by the editor in the present edition, with this exception, however: in the present edition credit to the several eminent contributors is given here in the preface.

Dr. Walter Wyman, Supervising Surgeon-General, U. S. Public Health and Marine-Hospital Service, has revised the chapter on "Quarantine." The chapters on "School Hygiene," "Exercise and Training," "Baths and Bathing," and "Clothing," grouped together with one chapter heading "Personal Hygiene," were prepared by Dr. Francis W. Upshur, lecturer on pathology, hygiene, public health and dietetics in the University College of Medicine, Eichmond, Virginia. The chapter on "Military and Camp Hygiene" was entirely rewritten throughout by Walter D. McCaw, Surgeon-Major, Medical Department, Surgeon-General's Office, Washington, D. C. The chapter on "Naval Hygiene" also was entirely rewritten by Henry G. Beyer, Major Surgeon United States Navy and professor of hygiene, etc., in the United States Army and Navy Medical School, Washington, D. C.

The other chapters were revised and supplemented by the editor. It is hoped that this edition will be favored with the same cordial reception accorded to former editions by students and teachers throughout the country.

A. Eobin.

Wilmington, Del.

(in)

Preface To The Third Edition

In this edition every chapter has been subjected to a careful revision, and the advances in sanitary science and practice have been incorporated.

Recent legislation in the United States and Canada has almost revolutionized quarantine practice. Surgeon-General Walter Wyman, and Dr. H. D. Geddings, of the United States Marine-Hospital Service, have, at the request of the author, entirely rewritten the chapter upon "Quarantine," and it will be found to represent fully the modern principles and practice of maritime sanitation.

Medical Director Albert L. Gihon, United States Navy, has again thoroughly revised the chapter on "Marine Hygiene."

With the view of making the book still more useful to teachers, students, and sanitary officers than heretofore, an analytical set of questions has been appended to each chapter, and a separate section has been added on methods of examination of air, water, and food. For these additions the author is indebted to Professor Seneca Egbert, of Philadelphia. Dr. Egbert has also carefully revised the chapter on "Vital Statistics."

The author desires to thank all who have assisted him in the work, and especially the sanitarians throughout the country who have been helpful in the way of criticism and suggestion. He hopes that the new edition will merit, as well as receive, the approval of all students of preventive medicine.

G. H. E.

Balximobe, Md.

(iv)

Preface To The First Edition

The aim of the author in writing this book has been to place in the hands of the American student, practitioner, and sanitary officer, a trustworthy guide to the principles and practice of preventive medicine.

He has endeavored to gather within its covers the essential facts upon which the art of preserving health is based, and to present these to the reader in clear and easily understood language.

The author cannot natter himself that much in the volume is new. He hopes nothing in it is untrue.

G. H. E.

Contents

Chapter Page

I.— Air 1

II.— Water 46

Chapter I. Air

Exact investigation into the influence of the atmosphere upon health is yet in its infancy. Enough has been learned, however, to show that changes in the composition of the air, in its density, its temperature, its humidity, its rate and direction of motion, and possibly its electrical or magnetic conditions, influence in various ways the health of the individual. It is only very recently that any scientific attempts have been made to trace the bearing of atmospheric changes upon health. The observations already recorded indicate that a thorough study of meteorological phenomena in connection with the origin and progress of certain diseases is a promising field of labor for the educated sanitarian. The meteorological observations which have been gathered by the United States Signal Service, together with elaborate studies made by the meteorologists and climatologists of other countries, already form such a large and tolerably complete and well-arranged body of facts, that reasonably accurate deductions can even now be made. Heretofore, in studying the sanitary relations of the atmosphere, both in this country and abroad, the attention of observers has been riveted almost exclusively upon the changes in its composition occurring within certain limited areas. It is, perhaps, equally important to study this universally diffused and necessary condition of vital activity in its broader and more general relations. It will be shown, in the course of the present work, that the meteorological features of countries, or of seasons, or even the daily atmospheric changes, exercise an important influence upon life and health. In order to fully appreciate these relations it will be necessary to first give a brief summary of the facts and laws of meteorology.

2 TEXT-BOOK OF 11VCI EN I \

THE COMPOSITION AND PHYSICAL CONDITIONS OF THE ATMOSPHERE.

Atmospheric air has the following composition : —

Oxygen 20.91 per cent.

Nitrogen 77.95 "

Argon 1.00

Carbonic acid 0.04

Aqueous vapor variable.

Traces of organic matter, ozone, mineral salts, ammonia, nitric acid, krypton, neon, metargon, carburetted hydrogen.

These proportions are maintained, with but very little change, at different heights. At first thought, it would seem that carbon dioxide, being much heavier than the other constituents of air, would accumulate in the lower regions of the atmosphere, and there cause an excess of this poisonous constituent ; but in obedience to the law of diffusion the intermingling of the component gases is perfect, and the proportion of carbon dioxide in the atmosphere is quite as great on mountain-tops as in the deepest valleys. This diffusion of gases, however, is modified by the wind and, in cities, by high buildings.

The proportion of nitrogen in atmospheric air is generally uniform, while that of oxygen varies, depending to a great extent upon the amount of carbon dioxide present. Hence, an increase in the amount of the latter constituent is usually accompanied by a diminution of oxygen, inasmuch as the formation of carbon dioxide can only take place at the expense of oxygen. The reciprocal activities of animal and vegetable life are beautifully illustrated hy these relations between the oxygen and carbon dioxide in the air. In the processes of combustion and oxidation, oxygen is withdrawn from the atmosphere, and combines with carbon, forming carbon dioxide. During vegetable growth, on the other hand, carbon dioxide is withdrawn from the air by the leaves of plants, and decomposed into its elements, carbon and oxygen. The carbon is used in building up the plant, while the liberated oxygen is restored to the atmosphere. The animal consumes oxygen, and gives out carbon dioxide; the plant resolves this compound into its constituent elements, and gives back the oxygen to the air. However, at night, or in the absence of sunlight, plants evolve carbon dioxide instead of oxygen, and for this reason it is injurious to keep plants in bedrooms over night.

The atmosphere extends upward from the surface of the earth to an indefinite distance. The limit has been variously placed at from 75 kilometres to 40,000 kilometres. For all sanitary purposes

COMPOSITION AND CONDITIONS OF ATMOSPHERE. 3

the former may be taken as the upward limit of the atmosphere. In obedience to the law of gravity, this mass of air presses everywhere directly downward — toward the earth's centre — with a force equal to its weight. If a column of this air be balanced by a column or mass of any other matter — the columns being of the same diameter — we have a relative measure of the weight of the atmosphere. The instrument with which the weight or downward pressure of the air is measured is called a barometer. The atmosphere, at the sea-level, presses downward with a force equal to the pressure of a column of mercury 7G0 millimetres high. Hence, the barometric pressure at sea-level is said to be 760 millimetres, or 30 inches. If the barometer be carried to the summit of a mountain 1000 metres above the level of the sea, or be taken to the same altitude in a balloon, the mercury in the barometer-tube will fall about 90 millimetres. These 90 millimetres of the mercurial column represent the weight of 1000 metres of air now below the barometer, and consequently not measured or balanced by it. 1

Upon ascending from the sea-level, it is found also that the air, being less pressed upon by that which is still above it, becomes more rarefied and lighter ; its tension, as it is termed, is less. Hence, for the second 1000 metres of ascent above the sea, the mercury will fall a less distance in the tube, the weight removed not being so great as in the first 1000 metres.

The following table shows the diminution in atmospheric pressure for every 1000 metres above sea-level: —

Sea-lev

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

10,000

11,000

12,000

15,000

20,000

Table I.

el 760.0 millimetres.

metres 670.4

591.5

521.0

460.3

406.0

358.2

316.0

278.8

1 The figures here given are not absolute, but merely approximate. The limits of this work do not allow a full discussion of the meteorological elements modifying the pressure of the atmosphere at sea-level.

Variations in temperature and humidity of the air influence the tension of the atmosphere in a marked degree, and affect the height of the barometric column. In fact, most of the changes of atmospheric pressure at the surface of the earth are directly due to changes in temperature and humidity. Increase of temperature diminishes the density of the air. Hence, when the temperature rises the pressure decreases.

The proportion of moisture (aqueous vapor), if increased, likewise causes a diminution in pressure. It is found, for example, that when the amount of aqueous vapor in the air increases, the barometer falls. This is due to the fact that the specific gravity of aqueous vapor is less than that of dry air, being in the proportion of .623 to 1.000. Hence, as aqueous vapor is diffused through the air, the latter becomes lighter, — or, in other words, the barometric pressure diminishes.

The warmth of the air is primarily derived from the sun. On a clear day about one-fourth of the heat of the sun's rays is given off directly to the air during the passage of the heat-rays to the earth. Of the remaining three-fourths, part is reflected from the earth, while the larger portion is first absorbed by the earth and then given off by radiation and convection to the superincumbent air.

The air is always warmer near the earth's surface on a clear, sun-shiny day; for, as soon as the earth gets warmer than the air immediately above it, the excess of heat is given off to the latter by convection and radiation. On ascending from the surface of the earth the temperature decreases, and on the summit of a high mountain the air is always colder than at its base. The decrease of temperature with the ascent equals 1° F. to every 300 feet.

Professor Tyndall has shown that dry air absorbs less heat than air which is charged with vapor. For this reason the sun's rays strike the earth with much greater intensity on a very dry than on a moist day, while on the latter a larger proportion of the heat-rays is intercepted before they reach the earth.

Recent experiments seem to show, however, that the difference in diathermancy between dry and humid air is not so great as supposed by Tyndall. The depth of the air-stratum, through which the sun's rays pass, is of greater influence than the humidity.

Air, at different temperatures, is capable of absorbing different amounts of aqueous vapor. Thus, air at a temperature of 4° will require a much smaller amount of vapor to produce saturation than air at a temperature of 30°. For this reason air which appears "damp" at the former temperature, both to the bodily sensations and

COMPOSITION AND CONDITIONS OF ATMOSPHERE.

to appropriate instruments, would be considered as "dry" at the latter temperature, although the actual amount of vapor present, or absolute humidity, is the same in both cases. 2 In meteorological observations for sanitary purposes, the relative humidity is the condition deserving especially careful study.

It must be borne in mind that the mere statement of the percentage of relative humidity, without taking into account the temperature of the air, is of little significance. A like remark is justified with regard to statements of absolute humidity, when used to illustrate the apparent effects of atmospheric moisture upon life and health.

The following table shows the absolute humidity corresponding to the same relative humidity at different temperatures. It also includes the total possible absolute humidity and the difference between the actual and possible humidity (deficiency of saturation) at the temperatures given: —

Table II.

Temperature °C.

Relative Humidity (per cent.).

Absolute Humidity (grammes per cubic metre).

Greatest Possible Absolute Humidity.

Deficiency of Saturation.

+10 20 30

60 60 60 60 60 60

In forests the relative humidity is usually higher than over unwooded districts, although the absolute humidity may be the same, or, perhaps, even less. The evaporation is usually much greater in the open air than in forests. In closed apartments the evaporation may be greater or less than in the open air, depending upon the local conditions present.

2 By "absolute humidity" is meant the total amount of vapor present in a certain mass of air. By the term "relative humidity" meteorologists designate the proportion of vapor present at certain temperatures, compared with full saturation of the air with vapor, which is reckoned 100. Thus, air which is saturated, or whose relative humidity is 100 at 4°, would have a relative humidity of only 24, if the temperature were raised to 27°, because jn the latter case the capacity of the air for aqueous vapor is increased. Relative humidity is always designated in percentages; absolute humidity in grammes per cubic metre or grains per cubic foot.

The motion of the air — wind — is caused by differences in presssure; the latter being due to differences in temperature and humidity. A mass of air traversing a large body of water absorbs vapor, unices already saturated, and becomes moist; if it pass over a wide tract of dry land it loses moisture and becomes dry. Therefore in the eastern portion of the American continent, an easterly or southerly wind, which comes from over large bodies of water, and which is usually warm, and thus capable of holding a large quantity of water in a state of vapor, is always moist. On the other hand, a northerly or westerly wind, coming over a large extent of dry land, and from a colder region, is nearly always a dry wind. On the Pacific coast these conditions are reversed; there a westerly wind is a moist wind, while an easterly wind is dry. The dreaded easterly wind of England is likewise a dry wind. It is probable that the direction and rate of motion of air-currents have considerable influence upon the origin or intensification of certain diseases.

The electrical and magnetic conditions of the atmosphere have been as jet studied to little advantage. It is only known that atmospheric electricity is, in most cases, positive, and that its intensity increases with condensation of vapor. There seems to be no doubt that the varying states of atmospheric electricity are closely connected with evaporation and condensation. There is reason to believe that a fuller knowledge on these topics will yield most important results to the student of hygiene.

Ozone, which is oxygen in an allotropic and highly active condition (0 3 ), is generally absent from town air; and when it does appear, as after a summer storm, it is in such insignificant amount as to have no influence on health.

BACTERIA IN THE AIR.

In localities which are free from human or animal habitation, as in open plains, high moutains, midocean, etc., the air is free from bacteria; on the other hand, bacteria will be present wherever man or animal abides. The number of bacteria will be in direct proportion to the density of the population, the larger number being found in cities, and, again, in the overcrowded portions of the large centres of population. Defective sanitation will increase the number of bacteria. In addition to bacteria, the air contains yeasts and the spores of moulds and of the lower fungi. The moulds are provided with fine spore-bearing filaments, which become detached and float in a

INFLUENCE OF ATMOSPHERIC PRESSURE. 7

free condition. The bacteria, on the other hand, do not float, but are carried by particles of dust or moisture. The epoch-making experiments of Pasteur and Tyndall have demonstrated, the axiomatic proposition that without dust there are no bacteria. Consequently, any disturbance which raises dust will also increase the number of bacteria in the air, and, conversely, any agent which allays dust also purifies the air. It is for this reason that the use of a damp cloth is preferable to dusting, and a carpet-sweeper is more sanitary than a broom. The bacteria present in the air may be of three kinds: (1) Harmless bacteria; (2) bacteria which produce putrefaction; and (3) bacteria which cause disease. The latter are the most important and are derived from sick persons. The actions of coughing, sneezing, speaking, and even of deep breathing distribute minute droplets of secretions from the respiratory passages and thus infect the atmosphere. Diphtheria, influenza, pneumonia, whooping cough, tuberculosis, and other infections of the respiratory organs may be and are communicated in this way. The fine particles of the bacteria-laden secretions may be directly inhaled by the person standing in front of the mouth of the patient, or else they fall to the ground, dry out, and dry bacteria are carried by the dust into the air. It is in this way that the sputum from consumptives becomes a serious and constant source of infection. In eruptive fevers, like measles, scarlet fever, small-pox, etc., the causative agent, still unknown, is eliminated through the skin and carried into the air by the fine particles of dry epithelium. The bacilli of t3'phoid fever and cholera may also find their way into the air through the drying of infected sewage or water, or the excreta from the patient may become mixed with dust, and, when dry, be carried into the air and subsequently deposited in water, milk, or other food. That infection by this method is not more frequent is due to the fact that many pathogenic bacteria are destroyed by drying and sun-light.

INFLUENCE OF CHANGES OF ATMOSPHERIC PRESSURE ON HEALTH.

The effects of a considerable diminution of pressure are familiar to every one in the "mountain sickness" which attacks most persons on ascending high mountains. M. Bert has shown experimentally that similar effects can be produced in an air-tight chamber by diminishing the pressure. The symptoms produced under a pressure equivalent to an altitude of from 4000 metres to 5000 metres were a feeling of heaviness, nausea, ocular fatigue, rapidity of pulse, convulsive trembling on slight exertion, and a sensation of languor and general indifference to the surroundings of the individual.

M. Lortet, who has left on record his experiences in the higher Alps, says that the symptoms noticed on ascending to high altitudes are: Labored respiration, increased rapidity of pulse, depression of temperature (as much as 4° to 7° C). The normal temperature was restored, however, after a brief rest. Still more severe symptoms have been noticed on ascending high mountains in South America and Asia. Aeronauts have lost consciousness, and in several instances life, on rapidly ascending to great altitudes. 3 According to the observations of the brothers Schlagintweit, distinguished explorers of the highlands of Asia, the effects of diminished pressure upon the human organism are: "Headache, difficulty of respiration, and affections of the lungs, — the latter even proceeding so far as to occasion blood-spitting, — want of appetite, and even nausea, muscular weakness, and a general depression and lowness of spirits. All these symptoms, however, disappear in a healthy . man almost simultaneously with his return to lower regions." A singular observation was made by these travelers on the effect of motion of the air upon the S3 T mptoms described. They say: "The effects here mentioned were not sensibly increased by cold, but the wind had a most decided influence for the worse upon the feelings. . . . When occupied with observations, we took. very little, if any, bodily exercise, sometimes for thirty-six hours; it would frequently occur nevertheless, even in heights not reaching 17,000 feet (about 5150 metres), that an afternoon or evening wind would make us all so sick as to take away every inclination for food. No dinner was cooked; the next morning, when the wind had subsided, the appetite was better.

"The effects of diminished pressure are considerably aggravated by fatigue. It is surprising to what degree it is possible for exhaustion to supervene ; even the act of speaking is -felt to be a labor, and one gets as careless of comfort as of danger. Many a time our people — those who ought to have served us as guides — would throw themselves down upon the snow, declaring they would rather die upon the spot than proceed a step farther." 4

3 MM. Sivel and CrocC-Spinelli, two aeronauts, lost their lives in this manner during an ascent from Paris, in April, 1875.

4 Results of a Scientific Mission to India and High Asia. By Hermann, Adolphe, and Robert De Schlagintweit, vol. ii, pp. 484, 485.

INFLUENCE OF ATMOSPHERIC PRESSURE. 9

These symptoms disappear when persons are exposed to these conditions for a prolonged time. Thus, in the Andes there are places 4000 metres above sea-level which are permanently inhabited; and in the Himalayas there are villages at a height of over 5000 metres constantly occupied. In this country, Pike's Peak, 4350 metres above the sea, has been occupied since 1873 by observers of the signal service. The men seem to become acclimated, as it were, and suffer little or no inconvenience from the diminished pressure after a time.

The minor- disturbances of healthy function produced by diminished pressure (within the limits of 4000 metres altitude, or 460 millimetres barometric pressure) are an increase in the pulse and respiration rate. This is probably due to the struggle of the organism to take up the required quantity of oxygen, which is reduced in proportion by the rarefaction of the air. For example, the proportion of oxygen at a pressure of 460 millimetres wouM be equivalent to 12.6 per cent, at sea-level, instead of the normal 20.9 per cent.

Paul Bert has shown by personal experiments in the pneumatic chamber that the increase in pulse and respiration rate is not due to the merely mechanical diminution of pressure, but to the deficiency of oxygen. Hence the physiological effects of high altitudes upon circulation and respiration are not purely physical, due to diminished pressure, but vital, and depend upon the change in the chemical composition of the atmosphere. The simple diminution of oxygen without reduction of pressure will produce similar though not identical effects upon the organism.

Above the height of- 4000 metres above sea-level (below 460 millimetres pressure) the profounder disturbances of function characterized as "mountain sickness" come on. Different individuals react in different degree to the morbific influences of greatly diminished atmospheric pressure (and coincident reduction of oxygen). Thus Glaisher reached an elevation of 11,000 metres (191.1 millimetres pressure) and returned to the earth alive, while Croce- Spinelli and Sivel perished at the considerably lower elevation of 8000 metres, equivalent to a pressure of 260 millimetres (7.2 per cent, of oxygen).

The sanitarian is most concerned about the effects of pressure of the atmosphere from 760 millimetres down to 460 millimetres (or up to an altitude of 4000 metres above sea-level). The climatotherapy of various diseases requires that the effects of variations of pressure between these limits should be carefully studied, The observations of Mermod and Jourdanet 5 have illustrated the common physiological effects of these circumscribed changes, while the experiences of therapeutists have established the fact very clearly that many cases of phthisis improve markedly in a rarefied atmosphere, provided, however, they are not subject to hemorrhages, in which case high altitude increases the liability to hemoptysis. Other obsarvers have also shown that the effects of diminished pressure are not always beneficial, and Dr. Loomis has warned against the sending of patients with heart disease to high altitudes. Whether the lethal effects that have been recorded in such cases are due to the increased activity of the heart and heightened blood-pressure from deficient oxygen, or as suggested by Dr. F. Donaldson, Jr., to dilatation of the heart-walls from diminution of external pressure, is as yet unsettled. 6

It is probable that the diurnal or accidental 7 oscillations of barometric pressure at sea-level have no appreciable influence upon the organism. The statement is occasionally met that patients subjected to grave surgical operations often do badly during low atmospheric pressure, and some surgeons never operate when the barometer is low or falling if they can avoid it. An inquiry undertaken by the writer in 1876, in which the excellent records of the Massachusetts General Hospital and the observations of the Boston station of the United States Signal Service for five years were used as the basis of comparison, resulted negatively. The deaths following operations done on days when the barometer was high' or rising were exactly equal in number to those following operations when the barometer was low or falling. Unfortunately, the investigation was never pursued to the extent of including other meteorological elements, such as humidity, cloudiness, precipitation, etc. The numerous studies of the relations of variations of pressure to the progress of infectious diseases have also failed to yield any fruits of value. ^Yhether the nerve-pains so frequently complained of, especially by elderly patients, during the progress of areas of low barometer, are due to the diminished pressure, or to the influence of some other meteorological factor, such as humidity or electrical condition, cannot yet be decided.

5 Jourdanet states that while the French and Belgian soldiers in Mexico had an accelerated pulse, the natives had a normal pulse. In Mermod's observations the average frequency of the pulse at St. Croix (1106 metres above sea-level) was nearly four beats greater than at Strassburgh (142 metres). The condition of the natives at the high settlements of the Andes and Himalayas has not yet been investigated with exactitude.

6 American Climatological Association, 1887.

1 Meaning the oscillation produced by storm waves.

INFLUENCE OF TEMPERATURE ON HEALTH. H

Increased atmospheric pressure, as noticed in caissons, tunnels, and mines, produces increase in frequency and depth of respiration, diminution in the number of beats and volume of the pulse, pallor of the skin, increase of perspiration "(although Smith states that this is only apparent and due to lack of evaporation from the surface), increased appetite, and more abundant excretion from the kidneys.

Among the distinctly pathological effects of increased atmospheric pressure , are rupture of the drum of the ear, pain in the frontal and maxillary sinuses, neuralgic pains, nausea, sometimes vomiting, and local paralyses. Dr. A. H. Smith 8 defines this collection of symptoms as "The Caisson Disease," and gives the following summary of its characteristic features: —

"A disease depending upon increased atmospheric pressure, but always developed after the pressure is removed. It is characterized by extreme pain in one or more of the extremities, and sometimes in the trunk, and which may or may not be associated with epigastric pain and vomiting. In some cases the pain is accompanied by paralysis more or less complete, which may be general or local, but is most frequently confined to the lower half of the body. Cerebral symptoms, such as headache and vertigo, are sometimes present. The above symptoms are connected, at least in the fatal cases, with congestion of the brain and spinal cord, often resulting in serous or sanguineous effusion, and with congestion of most of the abdominal viscera."

The measures to be adopted in preventing "Caisson Disease" are: (1) Working during short shifts, from 2 to 4 hours; (2) abundant supply of fresh air; (3) the use of electric light, so as to save the oxygen; (4) slow decompression, at the rate of one minute for every three pounds of pressure. The disregard of the last rule has recently caused several deaths among laborers working in the tunnels under the Hudson Eiver.

INFLUENCE OF CHANGES OF TEMPERATURE ON HEALTH.

Many of the derangements of health ascribed to high temperature are to a considerable degree due to other factors, prominent among which are high humidity, intemperance, overwork, and overcrowding. There can be little doubt, however, that the importance

8 The Physiological, Pathological, and Therapeutical Effects of Compressed Air, p. 47, Detroit, 1886.

of the high temperature itself can hardly be overrated. It has been generally accepted heretofore that a high temperature, together with a high relative humidity, is most likely to be followed by sun-stroke. A careful comparison ^in a series of deaths from sun-stroke in the city of Cincinnati in the summer of 1881 shows, however, conclusively that a very high mean temperature with a low relative humidity is more liable to be followed by sun-stroke than the high temperature when accompanied by a high humidity. The same series of observations also shows that the number of deaths was greater on clear days than on cloudy or partly cloudy days. 9 A corroboration of this result is found in the fact that sun-strokes very rarely occur on shipboard, at sea, where the relative humidity is always high.