Practical treatise on warming buildings by hot water, steam, and hot air on ventilation and the various methods of distributing artificial heat and their effects of animal and vegetable physiology to which are added an inquiry into the laws of radiant and conducted heat, the chemical composition of coal, and the combustion of smoke

Front matter

H.T.UATSK01:U, ARCHITECrURALliiNCINEERINC BO OK SELLER, -I)"

„ . LOJTDOU,WC'.-<- b

*

Preface

V deemed advisable to add to this Treatise such information on heating by Steam and by Hot Air, as shall place the whole subject of warming and ventilating buildings by the various methods now in use fully before the reader.

The great object which has been kept in view in this work has been to render the subject as simple and as practical as possible ; and with this view many things have been thrown into notes, which might otherwise have properly formed a portion of the text. But by this means the work is rendered more useful to practical men, who require only simple facts in the most concise form; while for those who desire further to investigate the subject, the copious references given in the notes will facilitate their inquiries, and enable them to test the correctness of the reasoning on which the conclusions are founded.

The earlier editions of this work contained various papers by the author, some of which have been pubHshed in the Encyclopcedia Metropolitana, and others have been read before the Institution of Civil Engineers, and the Society of Arts : and it has been thought desirable still to retain in the present edition these several papers, as they all

PREFACE, relate to subjects bearing intimately on the general question of warming and ventilating buildings, and therefore tliey possess an interest for all who desire to investigate the scientific principles of the subject which forms the main object of this •work.

C. H.

Part First

on -warming buildings by hot water. Introduction

Chapter I

Cause of Circulation of the "Water — Inclination of the Pipes — Necessity for Air Vents — Open and Close Boilers — ^Pressure of Water — Effect produced on the OLrculation by increased Height of the Pipe — Compression of Water — Branch Pipes — Yariations in Level of Pipes

Chapter II

On the Motive Power of the Water — On increasing the Motive Power — Telocity of Circulation — Circulation of Water below the Boiler — Direct and Reversed Circulation — ^Air Vents — Supply Cisterns — Expansion of Pipes, &c

Chapter III

On the Resistance by Friction — Relative Size of Main Pipes and Branch Pipes — Vertical Main Pipes — Small Connecting Pipes — Branch Pipes at different Levels— Stop-cocks and Valves — Their Use and proper Size — Their place supplied by Cisterns — Inconvenience of them — Remedies

PAOE y[{{ CONTENTS.

Chapter IV

Permanence of Temperatnre-Eates of Cooling for different sized Bodies— Proper Sizes for Pipes— Eelative Size of Pipes and BoHer— Various Eorms of Boilers, and tlieii- Peculiarities— Boilers heated by Gas- Objections against contracted "Waterway in Boilers- Proper Size of Boilers for any given lengths of Pipe— What constitutes a good and efadent Boiler— Durability of different materials for Boilers 62

Chapter V

On the Construction of Purnaces-Combustion dependent on Size of Eire-bars— Purnace-doors, and other parts of Eurnace-Proportionate Area of Eurnace-bars to the Fuel consumed-Confining the Heat within the Em-nace-Dii-ections for Building the Furnace for different Boilers-Advantage of large Furnaces- Modes of Firing— Size of Chimneys

Chapter VI

ESTIMATE OF THE HEATING STTEFACE EEQTjmED TO WABM ANY DESCRIPTION OF BUILDINGS.

Heat by Combustion- Quantity of Heat from Co^-- ' Specific Heat of Air and Water-Measure of Effect for Heated Ii'on Pipe-Cooling Power of Glass- Effect of Vapour-auantity of Pipe reqmred to warm a given Space-Time required to heat a Budding- Facile Mode of Calculating the quantity of Pipe requireu in any Building-Quantity of Coal consumed 102

CHAPTER Vn.

Various Modifications of the Hot-Water Apparatus- Kewley's Siphon Principle-The High-Pressui-e System-Eckstein and Busby's Rotary Float Circulator-Fowler's Thermo-siphon-Price's Improved Hot-water Boxes-Rendle's Tank System--Corbett s Trough System for Evaporation-Theory of Evaporation

CONTENTS.

IX

TAGE

CHAPTEE YIII.

General Summary of the Subject — Points requiring particular attention — Abstraction of Air from the Pipes — Vertical Alteration of Level — Effect of Elbows in compensating unequal Expansions of the Pipe — Different Floors heated by one Main Pipe from the Boiler — Eeduced Effect from Pipes laid in Trenches — Effect of Cold Cm-rents of Air in neutralizing Heating Apparatus — Cements for Joints — Sediment in Boilers — Use of Salt in Pipes to prevent Freezing — Deposition of Vapour in inhabited Eooms — Necessity for Ventilation — Construction of Drying-rooma .... 159

CHAPTEE IX.

ON HEATIKG BY STEAM.

Variableness of Temperature— Alterations in the Level of the Pipes— Eemoval of Condensed Water — Proper Steam Pressure to be Employed — Mode of calculating the Heating Surface required — Size of Boilers — Quantity of Water Condensed — Boiler Surface required— Size of Furnace-bars — Mode of supplying Water to the Boiler— Safety Valves, Gauges, and other Appendages required— Air Vents and Outlets for the Condensed Water

CHAPTEE X.

ON HEATING BY HOT AIll.

Sylvester's Cockle-Stove— Improved Cockle-Stove— Estimate of Heating Power — Hot-air Stoves with Projecting Plates, and their Heating Power — Bernhardt' s Hot-air Apparatus — Difficulty with Horizontal Flues —Multitubular Hot-air Apparatus — Hot-blast Appa-

X

CONTENTS.

CHA.PTEE XI.

ON THE LA.-WS AND PHENOMENA OF HEAT.

E tuliation and Conduction— General Law of cooling Bodies in Air and other Gases— Different Law for Conduction and Eadiation— Effect by Incidence of the Eays— Effects of Surface on Eadiation— Effects of Colour- Effects of Eoughness— Absorptive Power of Bodies for Heat— Conducting Power of Metals— Conducting Power of "Wood — Conducting Power of Liquids — Cooling Influence of Water and Air— Eeflective Powers of Bodies— Specific Heat of Bodies— Latent Heat — Spontaneous Evaporation — Heat and Cold by Condensation and Earefaction of the Air- Motion of Liquids influenced by Heat— Effect of Heat on Strength of Materials

CHAPTEE XII.

Experiments on Cooling

Part Second. Chaptee I

ON THE VABIOTTS METHODS OJ? WAEMING AND VENTILATING BUILIiINGS, THE COMBUSTION OF FUEL, ETC.

Early Methods of Warming Buildings— the Eomans ; their Stoves— Baths— Elues— Mode of preparmg theii- Eirewood— the Persian Method of Heating- Chinese Method of Elues— Method of the Ancient Britons— Invention of CHmneys— Burning of Coals in England— Early Writers on the Subjec^— Imlirovements in the Eorm and Construction of Stoves and Eireplaces , "

CONTENTS.

XI

PAGE

CHAPTEE II.

Forms of Fii-eplaces and Chimneys— Should be made to reflect Heat — Contraction of Chimney-breast — Hollow Hearths and Backs for Fireplaces — Eumford's Principles of Construction — Errors in the Construction of Stoves — Eegister Stove in a Case — Jeffrey's Stove — Franklin's Pennsylvania Stove — Cutler's Torch Stove— Sylvester's Eadiating Stove — Eussian and Swedish Stove— Candy's Stoves — German Stoves— Hot-ail- Stoves— Cockle Stoves— Dr. Nott's Stove— Dr. Arnott's Stove— Franklin's Vase Stove — Gas Stoves — Joyce's Stoves — Beaumont's Stove — Palmaise Stove 270

CHAPTEE III.

ON THE CHANGES PBODTJCED IN ATMOSPHERIC AIR BY HEAT, COMBUSTION, AND RESPIRATION.

Necessity for Ventilation — Constitution of the Atmosphere — Ventilation, its early Invention — Subsequent Inventions for Ventilating — Effects of Contaminated Air on the Human Frame — Effects of Climate on Health and Longevity — Cause of Miasmata — . Effects of excessive Moisture and excessive Dryness of the Atmosphere — Eespiration ; its Products and Effects — Impiu'e Air — Carbonic Acid Gas — ^Vapour from the Body — Effect of Diminished Pressure — Electric Condition of the Air — Production of Ozone — Decomposition of Extraneous Matter in the Air — Effects of Hydrogen, Carburetted Hydrogen, and Carbonic Oxide — Quantity of Air required for Ventilation — Importance of the Air to Animal and Vegetable Life 297

XII

CONTENTS.

PAGE

OHAPTEE IV.

ON THE VAEIOUS METHODS OF PEODUOING VENTILATION.

Spontaneous and Mechanical Methods of Ventilation — Cause of Motion in Spontaneous Ventilation — Velocity of Discharge — Effects of unequal Height of Discharge Pipes — Defects in Ventilation of Churches — Proportions of Induction and Abduction Tubes — Quantity of Air discharged through Ventilators — Ventilating by Heat and by Chimney Draughts — Mechanical Ventilation by Pans, Bellows, and Pumps — Quantity of Air Discharged by these Means — Calculations of the Power expended 336

ON THE OHEMIOAL CONSTITUTION OF COAL, AND THE COil-

Early Use of Coal in England — Chemical Composition of Coal — Analyses of Coal — Combustion of Coal — Loss by imperfect Combustion — Loss from the Escape of Smoke — Loss by Carbonic Oxide — Causes of imperfect Combustion — Theory of Combustion — Temperature required for Combustion — Effects of Rarefaction of the Air — Effects of Hot Air — Quantity of Air required for Combustion — Methods of admitting Air — Combustion of Anthracite Coal — Description of various liansfor consuming Smoke — Ai-tificial Fuels 367

Appendix 412

CHAPTEE V.

On the Theory of Gaseous Efflux

CHAPTER VI.

BTJSTION OF SMOKE.

Introduction

The practice of employing hot water, circulating through iron pipes, for diffusing artificial heat, is an invention of acknowledged utility; and the present extensive and extending use of this invention renders it extremely desirable that its principles should be clearly defined, and the rules for its 23ractical ajDplication laid down with precision. Without this knowledge its success will be uncertain, its application limited, and its results imsatisfactory.

It can scarcely excite surprise, that prejudices should formerly have existed against this invention, while its merits and its principles were alike imperfectly known. Even at the present time they are but partially understood; and, therefore, to investigate these two subjects is the proposed object of the present treatise, with the view of facilitating its application, and extending the sphere of its utility.

There is_ scarcely any branch of science, or of art, in which an acquaintance with the laws of Nature does not enable us to derive greater advantages in its application than we could otherwise possess. There is no art, however humble, in which a knowledge of the laws that regulate matter does not open a wide and extensive field of useful improvement ; and no man can hope to

INTRODUCTION.

advance tlie sphere of useful invention, in any considerable degree, without some knowledge of the principles on which it depends. But, havmg this knowledge, he may, to use the words of a well-known writer, "if he have only a pot to hoil, be sure to learn from science lessons which will enable him to cook his morsel better, save his fuel, and both vary his dish and improve it.'' Although it is true that we are still ignorant of the more subtile agents which exist in the vast chain of causation, the laws which regulate the various phenomena of Natm-e are sufiaciently known to afford the most beneficial assistance to every branch of the arts and sciences : and the most recondite of scientific discoveries, as well as the most valuable inventions and improvements in the arts, are not more demonstrative of the truth of this assertion, than those which are the most simple and inartificial.

For an illustration of the utility of this Imowledge, we may refer to the law of gravity, not only because it is, of all natural phenomena, the most constant in its operation, and the most universal in extent, but because its influence is closely connected with the present subject of inquiry.

That all falling bodies gravitate with the same velocity, and, therefore, descend through a certain definite space in a given time, is, we know, an effect^ of which gravity is the cause. It is on the operation of this invariable law that many of our most valuable inventions depend. Its influence is equally exerted on all objects, as well the most mighty as the most minute. It is this which gives stability to the grandest productions of Nature, as well as to the most minute or artificial of our own works. It is fi'om this cause that we obtain the unerring action of our pendulums

INTRODUCTION.

and clocks ; and it is by this we obtain the circulation of hot water, with which we warm our dwellings. But by a knowledge of the cause of these eifects, of the extent of its operation, and of the laws by which it acts, we can, by varying the cu'cumstances of a gravitating body, alter also the velocity of its descent. We accomplish this by bringing other causes into operation, which modify the result, notwithstanding the immutability of the laws of gravity : and thus, by a knowledge of the physical laws, we can modify and subject to oiu- will one of the most constant and universal agents in Nature.

The study of the laws which govern natural phenomena — which in all cases are so simple, so beautiful, so perfect — is, therefore, one of the most fruitful sources of inquiry which the mechanician can pursue. It opens to him new fields of useful inquiiy ; new applications of known inventions; new and simpler means of accomplishing known effects. And while it points to improvement in every dii-ection, it restrains the judgment fr'om false principles; for where it exists not, it is almost certain that the plans of the mechanician will either be only modified copies of existing inventions, or they will degenerate into wild speculations, unsupported on any reasonable foundation.

This is particularly observable in the case before us. The numerous failures which have occurred in the practical application of the invention of heating buildings by the circulation of hot water, are all distinctly referrible to the want of this kind of knowledge, and not to the object aimed at being itself unattainable. For whenever the physical laws are intended to be employed as the principal agents in producing any mechanical effect, it is an indispensable con-

B 2

INTRODUCTION.

dition that simplicity of action be kept in view. While it may farther be observed, that the endeavours to trace and elucidate the operating causes of the various phenomena, which occur in the course of practical experiments, are the sm-est means of facilitating original discoveries, as well as of promoting new adaptations of recognized principles.

The origin of the invention of employing hot water for diffusing artificial heat appears to be hid in considerable obscurity. It is not improbable that, like many other discoveries, it has been reproduced at various periods. It seems, however, to have been first used in France, by M. Bonnemain, in the year 1777, and was employed by him dm-ing several years for hatching chickens by artificial heat. The French Revolution, Avhich followed shortly afterwards, put a stop to this, as well as many other useful and scientific inventions, in that country ; and for several years the invention appears to have been entirely dormant; nor, indeed, does it appear ever to have been used by M. Bonnemain except for the pm-pose above mentioned. About the year 1817, the Marquis de Chabannes introduced a similar apparatus into this country for heating a conservatory, and also heating some rooms in a private house, by pipes leading from the kitchen boiler. In the folloAving year he published, in London, a paniphlet describing thl^ apparatus, and some ingenious modifications of hot-air stoves. The invention appears to have made very little progress for some years. In 1822, Mr. Bacon, a gentleman of fortune, introduced the use of hot water into his forcing-houses, using for the purpose a single pipe of large diameter, communicating with the boiler ; and, by giving a slight elevation to the pipe from the horizontal line, he was thus enabled to produce a

INTEODUGTION.

cii'culation of the water, the hot water slowly passing along the upper part of the nearly horizontal pipe, and the colder water returning to the boiler along the lower part of the same pipe. The circulation in this apparatus was very imperfect ; and Mr. Atkinson, an architect, almost immediately afterwards suggested the addition of a second pipe to bring back the colder water to the boiler ; and thus at once the apparatus assumed the form which it has ever since retained. By this alteration the apparatus was brought very nearly to the same form as that contrived by M. Bonnemain more than forty years before, the principal difference being that M. Bonnemain used only very small pipes of gun-barrel size, while Mr. Atkinson used pipes of four or five inches diameter.

The honour of this invention has been claimed for Mr. Watt, prior to the time of M. Bonnemain using it in France ; but there appear no grounds for supposing he ever employed it without the intervention of steam, as a distributor of heat by circulation, in the manner in which it is now used. The mere motion of hot water in pipes is an invention of far greater antiquity than the time either of Watt or Bonnemain. Seneca has accurately described the mode of heating the water in the Thermse of Rome, of which Castell has given drawings ; * and which show that the method of heating baths by passing the water through a coil of pipes which passed through the fire was known and practised previous to the Christian era : and, except that the tubes were of brass, instead of iron, they were precisely similar, both in form and arrangement, to those used at the present day for the like purpose ; the lapse of nine-

* Castell' 8 "Illustrations of the Villas of the Ancients," p. 10.

INTRODUCTION.

teen centuries having apparently added nothing to our knowledge on this subject.

Since the first introduction of the hot- water apparatus for warming bu.ildings, the variations made in its more complicated an-angements appear to have been very gradually adojpted. Each time that an apparatus has been erected, the experimentalist has deviated in some small degree from the model of that which preceded ; apparently afraid of venturing on too great a variation, 5^et requiring, from contingent circumstances, some alteration of its form and application. This mode of proceeding, though natural while the principles were not thoroughly understood, has frequently led to both inconvenience and loss, in consequence of the numerous failures to which it has given rise, by unintentional deviations fr'om the principles. In the present attempt to elucidate the subject, it will, however, be shown that success needs not be uncertain, provided only that the laws of physics be justly applied and strictly adhered to.

So numerous have been the failures which have occurred in this method of heating buildings, that nothing but the intrinsic merits of the invention could possibly have made it retain its hold on the public favour. Every imaginable kind of mistake has been made in apparatus erected on this plan. And these mistakes still continue to be made as frequently' aS ever, notwithstanding the vast number of buildings in which it has been successfully applied, and which might be consulted for correct information.

Neither the capabilities of this method of warming, nor the various usefal purposes to which it is applicable, are even yet fully appreciated. There are no buildings, however large, to which it cannot be advantageously adapted, nor any that pre-

INTKODUCTION.

sent insui'mountable difficulties in its practical application. And in many useful purj)Oses connected "witli arts and manufactures, it can be most advantageously employed, though its apjjlication to these purposes has hitherto been greatly overlooked. Its merits, however, will best appear by the j)lainest statement of facts.

Since the publication of the first edition of this work, in 1837, amjDle opportunities have occurred for- testing in every variety of form the accuracy of the rules and calculations which were there given, for constructing and apportioning the hot-water apparatus to the varying circumstances under which it is applied. The most important of -the calculations are those on the heated surface required to warm a given building, and the proper size of the boiler and the fui-nace. The data on which were founded the rules, and tables for calculating these proportions, were carefully compared, both experimentally and practically ; and the extensive use which has been made of these rules, with perfect success, leaves no doubt whatever as to their complete accuracy.

The object which has been aimed at has been to render the work as clear as possible to practical men, while, at the same time, it should not be below the notice of those who might desire either to acquaint themselves with the scientific principles of the invention, or with the general bearing of the subject, on the health, the comfort, the physical development, and even the duration of life, of a large portion of the human race. The effects of unwholesome air on the animal economy from imperfect ventilation and deleterious methods of producing artificial warmth, is a subject which has hitherto excited far too little attention. The laws which connect man with the physical universe are of far higher generalization than many are

INTKODUCTION.

inclined to believe : they bind together not alone the phenomena of our own world, but of the entire material universe ; and yet mankind vainly suppose they can act as they please with relation to these immutable laws, — that they can fashion them to their own will, and to their own imagined wants; and instead of endeavouring to make themselves more thoroughly acquainted with these laws, devised by unerring Wisdom for the imiversal good, they choose rather to act either in direct opposition, or, at least, in total neglect of those great truths which philosophy has been permitted to discover and unfold. But it is certain that they cannot with impunity neglect the great truths which physiology discloses ; nor can they place themselves in opposition to those fundamental laws which bind together the whole physical universe, without entailing upon themselves the jDcnalties which the Great Author of those laws has made the inseparable condition of their infringement.

It is hoped the following pages will place this matter in a clear point of view, in a sufficiently popular manner.

A PEACTICAL TREATISE,

ETC.

PAET FIEST.

ON WARMING BUILDINGS BY HOT WATEE, AND ON THE LAWS OF HEAT, ETC.

Chapter 1

Cause of Circulation of the Water — Inclination of the Pijpes — Necessity for Air- vents — Open and Close Boilers — Pressure of Water — Effect produced on the Circulation by increased Height of the Pipe — Compression of Water — Branch Pipes — Yariations in Level of Pipes.

(Art. 1.) In endeavouring to explain the principles of the various forms of apparatus in which hot water, circulating through iron pipes, is employed as a means for distributing artificial heat, the first object should be to point out, as clearly as possible, the power which produces the circulation of the water; for without a clear perception of this part of the subject, there will always be an uncertainty as to the results which will obtain, when any departure is made from the most simple form and arrangement of the different parts of the apparatus. It is this circulation which causes all the water in the apparatus to pass successively through the boiler, and then communicates the heat that is thus received from the fuel to the

CAUSE OF CIRCULATION.

various buildings or apartments which it is designed to warm. Without this circulation, those parts of the apparatus which are remote from the fire would not receive any heat ; because water is so bad a conductor that it is only when there exists perfect freedom of motion among its particles, that it acts at all as a conductor of heat, so far, at least, as regards any practical and useful effect. It is in a complete and perfect circulation, therefore, that the efficiency of a hot-water apparatus depends, and that the greatest amount of heat is obtained by it fr-om a given quantity of fuel.

(2.) The only treatise hitherto published, in which any attempt has been made to explain the cause of the cfrculation of the water in this description of apparatus, is Mr. Tredgold's work on heating by steam ; and the effect is there referred entirely to an erroneous cause. In the Appendix to that work, the cause of motion is thus explained.

"If the vessels a b, and pipes, be filled with water, and heat be applied to the vessel A, the effect of heat will expand the water in the vessel a, and the sm-- face will, in consequence, rise to a higher level, a d, the former general level sm-face being h h. The density of the fluid in the vessel A will also decrease in consequence of its expansion ; but as soon as the column c d (above the centre of the upper pipe) is of a greater weight than the column / motion will commence along the upper pipe from a to b, and the change this motion produces in the equilibrium of the fluid will cause a corresponding motion in the lower pipe from B to A."

(3.) Now it is certain that this theory will not account for the circulation of the water, under all

B

CAUSE OF CIRCULATION.

circumstances, and every variety of form of the apparatus ; and as the cause of motion must be the same in all cases, any explanation which will not apply universally must necessarily be erroneous. Were this the true cause of motion, there would be no difficulty in obtaining a circulation in all cases ; for, according to this reasoning, whenever the level of the water is higher in the boiler than in the pipes — or even if an upright pipe were placed on the top of a close boiler, by which the pressm-e on the surface would be increased — the water must of necessity circulate through the pipes : while, on the other hand, if this hypothesis were correct, the water in an apparatus constructed as in the following figure would not circulate at all.

(4.) Suppose the apparatus, fig. 2, to be filled with cold water, and the two stop-cocks, / to be closed : on a23plying heat to the vessel a, I'lf*- 2.

the water it contains will expand in bulk, and a part of it will . flow through the small waste pipe which is so j)laced as to prevent the water rising higher in the vessel A than the top of the vessel b. The water which remains in the vessel a, after it has been heated, and a portion of it has passed through the waste-pipe will evidently be lighter than it was before, while its height will remain unaltered. Suppose, now, the two cocks, / to be simultaneously opened ; the hot water in the boiler a will immediately flow towards b through the upper pipe, and the cold water in b will flow into A through the lower pipe ; although, by the hypothesis previously alluded to, unless the water in the vessel a, above the pijDe c, were heavier, or rose to a higher level than the water

CAUSE OF CIECULATION.

in the vessel b, no circulation could take j)lace. In this case, therefore, we must find another explanation of the cause of motion,

(5.) The power which produces circulation of the water will be found to arise from a different cause than that which is here stated ; for we see that this reason is insufficient to account for the effect, even in one of the simplest forms of the apparatus.

In order to explain this, let us suppose heat to he applied to the boiler a, fig. 2. A dilatation of the volume of the water takes place, and it becomes lighter ; the heated particles rising upwards through the colder ones, which latter sink to the bottom by their greater specific gravity, and they in their turn become heated and expanded like the others. This intestine motion continues until all the particles become equally heated, and have received as much heat as the fuel can impart to them. But as soon as the water in the boiler begins thus to acquire heat, and to become lighter than that which is in the opposite vessel b, the water in the lower horizontal pipe d is pressed by a greater weight at z than at y, and it therefore moves towards a with a velocity and force equal to the difference in pressure at the two points y and 0.* The water in the upper part of the vessel B would now assume a lower level, were it not tliat the pipe c furnishes a firesh supjDly of water from the boiler to replenish the deficiency.

* To any person unacquainted witli the science of Hydrostatics, this may probably appear erroneous, because the quantity of water contained in a is much greater than that in b. It is, however, one of the first laws of Hydrostatics that the pressure of fluids depends for its amount on the height of the fluid only, and is wholly irrespective of the bulk, or actual quantity of fluid : therefore, a pipe which is not larger than a quill will transmit the same amount of pressure as though it were a foot, or a yard, in diameter, provided the height be alike in both cases. (See Art. 10.)

INCREASED VELOCITY OF CIRCULATION. 13

By means of this unequal pressure on the loweipipe, the water is forced to circulate through the apparatus, and it continues to do so as long as the water in b is colder, and therefore heavier, than that which is in the boiler ; and as the water in the pipes is constantly parting with its heat, both by radiation and conduction, while that in the boiler is as continually receiving additional heat from the fire, an equality of temperature never can occur ; if it did, the circulation would cease.

(6.) We see, then, that the cause of the circulation is the unequal pressure on the lower pipe of the apparatus ; and that it is not the result of any alteration which takes place in the level of the water, as has been erroneously supposed. Indeed, the truth of this appears so plain, that it would scarcely require explanation at such a length, were it not that false opinions in this matter appear to have led to many errors in practice.