Abbildungen der Seite




183 made to the quantity of deposit being bring the question within narrower and so interposed between the plates and the better defined limits. water.

Hitherto, I have examined the subject In another case, from a deficiency of with reference to the illustrations which water (through design or neglect), the practice presented : I will now draw boiler exhibited the ordinary appearance some from the statements of others. of having been overheated : some plates “If a metallic bar," says Professor were softened, bulged, and ruptured (one Brande, speaking of conduction, “be of which plates I have now in my pos- placed in connexion with a constant session), and the seams and rivetings, source of heat, and we wait till it has not only along the bottom, but extending taken up a permanent state of temperato the sides and crown, were deranged, ture, we shall find that for distances requiring new riveting and caulking from the source, taken in arithmetical Although accidents of this sort are of progression, the excess of temperature daily occurrence in the manufacturing above the surrounding medium, will districts, the present was attributed to form a geometrical progression.” some imaginary expanding and contract We have here à defined connexion ing influence, under an ingeniously sup between those rates of progression, and posed alternate heating and cooling pro permanent state of temperature" in cess; for notwithstanding the entire bot the conductor bar.

Now this per: tom and flues were exposed to an uniform manentstate corresponds with what I stream of heated products of combustion have termed the statical heat of the bar, from the furnace, the theory assumed and which indicates the degree in which that there was a body of air at one time the metal will be affected by heat, indriving the flame against the boiler bottom juriously or otherwise. When Mr. and causing it to expand ; and again, Brande uses the term

permanent that the same air caused the same part state,” it is not to be taken as referring of the boiler suddenly to contract, until to any particular temperature, but merely the rivets were dragged in opposite direc to the condition (as to temperature) in tions (like a man attempting to pull his which the conductor may then be placed; own arms off), and the boiler so became and only cæteris paribus, as regards the leaky! The ingenuity of this mode of surrounding state of things; inasmuch making the boiler leaky, might however as each, and every change, will induce a have been spared, had the engineer for a new and varying state, or statical temmoment considered, that, as this boiler, a perature. This, however, will be more new one, had been leaky from the begin apparent as we proceed. ning, and even to a considerable extent; The point now under consideration is and there was no water gauge for exhi this, how far the nature of the recipient biting the height of the water within, the will influence this permanent state of deficiency which led to the overheating statical temperature ? The bar and its and injury, might, without any great state, as mentioned by Mr. Brande, we stretch of fancy, have been occasioned see had reference solely to one kind of in this natural way. The air, in this recipient for the conducted heat, namely, instance, no doubt, was "crude air" the air. If, however, it be brought into (vulgo, pure air), and doubtless would connexion with a different class of recinot have produced such dire effects had pients, as oil, mercury, or water, a new it been "diluted with nitrogen and steam,” and different pro tempore, though “peras recommended by Mr. Armstrong, to manent state," will be established. In whom the above ingenious theory of ex other words, the statical heat will vary panding and contracting is attributed. as the circumstances which govern it,

When practical men will thus strain and which I am endeavouring to show after new and speculative sources of in are solely attributable to the nature and jury, while they overlook natural and properties of such recipient. ordinary causes, it is time that further What then are the circumstances which inquiry be made, and the subject taken modify or govern the statical heat ? I out of the hands of quacks and pretend here prefer using the term statical, ers. A closer view of the principles rather than permanent, as it avoids conwhich practically govern the conduction fusion, and, without any apparent conof heat through metallic bodies, will help tradiction, involves the idea of a temto clear away those erroneous notions, and perature, though still defined, yet varying

according to circumstances. This stati Practically speaking, then, this distinc. cal heat, then, must be considered under tion is peculiarly applicable to our pretwo relations, namely, as indicating, first, sent inquiry, the whole turning on this the amount of power exercised by the point, whether conduction or convectica recipient in carrying away the heat re be the prime mover in producing those ceived from the conductor, and secondly, fluctuations of temperature in the conthe amount of influence which such ductor which lead to useful or injurious power exercises on the conductor itself. results. Let us now examine, practically,

With respect to this direct connexion the effect of this convection, or power of I find but little notice has been taken. carrying away the heat, and the extent Professor Kelland, of Cambridge (now to which it influences the heat conducted, of Edinburgh), has however, in his both as regards quantity and rapidity. “ Theory of Heat," distinctly recog Let us suppose that a given quantity nised it as demanding attention. Under of heat is passing through a metallic bar, the head “ Convection," he observes, with a rapidity due to its maximum “A mode of loss of heat analogous to power of conduction ; and that it is taken conduction, is that to which Dr. Prout up, or absorbed, by the recipient, with has applied the term convection. When a corresponding rapidity. The temperaa hot body is in contact with the air, ture of the conductor will then truly inthe part next the body becoming more dicate that permanent state, referred to elastic (rarefied), flies off and is sup. by Mr. Brandc. If, however, this abplied by colder portions; thus the heat sorbing power on the part of the recipient of the body is conveyed away more be diminished, the rate at which the heat rapidly than it would be if the air passes from the conductor will also be were not in motion. It is obvious that diminished in the same ratio ; and as a this circumstance will materially affect necessary corollary, the rate of the cur. all experiments on the motion of heat, rent of heat through the conductor will in which it is hardly possible to estimate be reduced in a corresponding degree. the effects due to this cause.” Now it is The practical question then is, to what this very circumstance, as it affects the extent will this "

permanent state " or condition of the metals employed in eva statical temperature of the conductor be porative processes, that I am desirous of affected by such diminished power of examining; inasmuch as this “

convection in the recipient ; for this station,' or carrying away, as the term tical heat will ever influence the question imports, will, in practice, be found to be whether such conductor be under or overthe primary source of good or evil. heated. The following experiment will Conduction being but the secondary, or illustrate some of the relations which induced cause.

affect this inquiry. Professor Kelland, further on, in ex The annexed engraving, it will be amining the mode by which heat is seen, exhibits a state of things, corres. transferred from one part of a body to ponding in principle, to that referred 10 another, observes, “When speaking of in your Number 967, in which the ther. solids, this is called conduction. It is mometer indicated the statical heat of clear, from the term itself, that we do the conductor bar. I have now extended not include, under this head, the transfer the illustration by lengthening the bar of heat by radiation; nor do we in and introducing threc thermometers, clude that transfer which takes place thus, to indicate the heat at three differamongst the particles themselves, carry ent sections, and mark more accurately ing with them the caloric they have ac the varying temperatures. quired, which we designate convection." In this engraving, as in the former It is manifest from this clearly defined one, let A represent the conductor bar; distinction between conduction and con B the vessel to contain the recipient, vection, that as the former refers to water, or air, or whatever it may be; C solids, so the latter refers to fluids the cock for letting off the liquid, when aeriform or liquid ; inasmuch as the employed as a recipient; D1, D2, D3, “ carrying with them " the caloric they the three thermometers, indicating the have received, involves a mobility among statical heat at their respective portions the particles which is inconsistent with of the bar ; E the constant source of heat, the nature of solids, wbile it is a correct being that from a powerful gas-burner, definition of that of fluids.

furnished with a metallic dome, on the

con vec



[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed]

Time in Therm.
minutes. No. 1.

No, 2.

No. 3.



principle of the “Solar Lamp;" and F, on the water being withdrawn, the ther. the protecting shield. To these I have mometer, nearest to the recipient and now added the chamber G, which was farthest from the heat, rose with the filled with charcoal powder, to act the greatest rapidity, indicating a species of part of a non-conductor. By this means, revulsion, or backing up of the heat ; and radiation from the bar was prevented; proving that the increased heat of the bar the thermometers received heat alone was referable to the diminished power of from their points of contact with the bar the recipient, and not to the supply of -the internal streams of conducted heat heat. This is extremely instructive, being confined, as though it were water proving the diminished power of convecin a canal or tube.

tion in the recipient air, comparatively By the following table, No. 1, we find with that of water. that in fitty minutes the water in the

TABLE I. vessel B was raised to the boiling point,

Therm. Therm. Temperature by the issue of what I have called the

of water. stream of dynamical heat, from the end 0 54 54 54

52 of the bar at A. A “permanent state' 5 164 126 98


10 of temperature was then attained; the

244 183 136


15 thermometers at their respective distances

300 222 166 118

20 340 254 189 from the source of heat, in arithmetical

25 366 274 206 168 progression, indicating the temperatures,

30 382 288 218 184 in geometrical progression, (or suffi

35 392 296 227

198 ciently near for our purpose, for accuracy

40 398 304 234 210 in this respect was not attempted), of

45 399 306 237 212 400°, 309°, 242,-the extreme differ



boiling ence being 158°. Let us now suppose 55 400 310 this to be the state of things in which the

60 413 340 284 The water maximum power of conduction of the 65

438 374 320 being run metal was brought into action. It is here 70 454 394 340 off, and the manifest that the amount of statical heat 75 464 402 350 recipient in the conductor will bear some ratio to

472 412 380 changed to the joint power of conduction in the bar 85 476 415 362 | air. and convection in the recipient water.

90 476 415

362) On the recipient being changed, by By inspection of this table, No. 1, we letting out the water and letting in the find that between the periods, 55 and 60 air, a new state of things is induced. minutes, (say in five minutes), the ther. The power of convection will be dimin

mometer No. 3, (the farthest from the ished, (air being a worse-recipient than heat), had risen 42°, that is, from 2120 water); the quantity of heat received to 284°; whereas, No. 1, (that nearest from the conductor in each unit of time to the heat), rose but 13° in the same will also be diminished; and the rate at five minutes, say from 400° to 413°; thus which the stream of conducted heat passes proving, that what may be called the through the bar will be influenced in a wave of statical increase had flowed backcorresponding degree.

The source of wards towards the source of heat, and in heat, however, remaining constant, the a manner strictly analogous to what would inevitable result is that accumulation take place if, instead of a stream of heat, will take place in the bar, and the thermo it had been a stream of water in a tube or meters instantly indicate an increase of canal. statical heat. This I believe to be the The bearing, practically, of this illusrationale of the process, and this is ex tration is, that the temperature of a conactly what we find confirmed in practice. ductor plate or bar will be influenced, not Now, if the velocity of the convection of so much by the quantity of heat imparted the new recipient, air, be ascertained, as to the conductor, as by the absorbing or well as the conductive power of the metal convecting power of the recipient. This bar, and both taken as constants, we shall also shows the practical error of attribe enabled to approximate to the amount buting injury to what is going on outside of statical heat in the conductor, and the the boiler, or in the plates themselves, degree in which the metal will be affected, while we neglect what is taking place

It is worthy of notice here, that con within the boiler, where the real source trary to what might have been expected of injury exists.



187 This table further shows, that, by rea mained stationary, such being the indi. son of the restricted power which the air cated amount of statical heat. (It should possesses of carrying away the heat from here be observed, that these numbers the conductor bar, the temperature of the cannot be taken as giving the absolute Latter rapidly increases, until a second temperature of the bar, although they "permanent státe is established ; and approach sufficiently near to give the rewhen the three thermometers stand, re lative temperatures, under the influence spectively, at 476', 415°, 362°, the of the recipients, air and water. It will Extreme difference here is but 114', hereafter be shown how near they apwhereas with water as the recipient it proach to such absolute temperature.) was 158. This relation of 114° to 158°, The recipient being now changed from then, indicates the ratio of the conductive air to water-the latter being poured into powers of the bar brought into action by the vessel B-we see the superior conthe influence of the respective recipients, vecting power of the latter at once air and water. Again, we see that this brought into action, by the immediate Det statical heat of the bar is in the ratio lowering of the temperature of the bar. of the convecting power of the recipient, On the water being again raised to the the thermometer nearest to the source of boiling point, a permanent state is again heat rising from 46° to 476, while that established, as in the preceding example, most removed rose from 242° to 362°. whatever variance has taken place being Many other instructive relations might accidental. It is here important to obhere be noticed, if time and space per serve how the analogy between the curmitted.

rent of conducted heat and a current of The more accurately to observe the water is maintained; for we see that, alvarying relations between the statical though thermometer No. 3 arrives at its and dynamical heat, under a different minimum, 228°, in 60 minutes, as marked state of things, I reversed the experi with an *, No. 2 does not reach it until ment, and began the operation with air as after 70 minutes, and No. 1 until 75 mi. the recipient; then changing it to water, nutes. These experiments were made 50 soon as the first permanent state of with great care, although the adjustment ternperature was established, and which, of the apparatus required much exactness, by the Table No. II., we see took place to produce uniform results. Throughout in forty-five minutes. The results here the whole we see sufficient to justify the are equally interesting and instructive. observation, that much remains yet to be TABLE II.

done, before this complicated subject be

exhausted. Time in Therm Therm, Therm.

The practical inference I

draw from the above experiments is, that 0 54 54 54

they afford sufficient proof of the position 5 170 132 100

before stated, namely, that it is not to the 10 270 220 178

The reci.

furnace or draught, or activity of the fire, 15 350 288

237 20

we are to look for that accumulation of 400

pient being
355 280
air, & then

heat in the plates of ordinary boilers 25 434 367

311 30

changed to

which produces overheating and rupture, 457 390 333


but to the recipient, and its powers of 35 471 406 349

carrying away and absorbing the heat 475


Temperature which the conductor plate or bar is capa45 476 414 362

ble of imparting. 50 476 414 362

54 55 432 315 230 127

Purposing to continue this subject on 60 388 298



a future occasion, 65 380 296 232


I am yours, &c., 70 374 *296

C. W. WILLIAMS, 236 186 75 *372 297 239 200

Liverpool, Feb. 21, 1842. 80 373 299 241 85 380 303 243 211

FURNACES AND BOILERS-MR. ARMSTRONG 90 388 306 244 212 95 392 309 244


Sir,-I had written a reply to and refutaThe three thermometers, we see, con tion of Mr. C. W. Williams's funny review tinued rising uptil they arrived respect of my “diffuso-separative" theory, as he ively at 476°, 414°, 360°, when they re calls it, in No. 963 of your Magazine, before

tinutes. No. I.

No. 2.

* No. 3,


of water.



« ZurückWeiter »