October 1, 1890.] 



KNOWLEDGE 



233 



When a moving body is brought to rest by such forces 

 as friction its energy seems to disappear — no equivalent of 

 potential energy seems to be produced, and it was long 

 thought that the work done against friction was lost. It 

 is only in comparatively recent times that it has been 

 recognised what becomes of the energy which is thus lost 

 sight of, and that the principle of the conservation of 

 energy has been established. 



Professor Clerk Maxwell has stated the principle of the 

 conservation of energy in the following concise form : 

 " The total energy of any body or system of bodies is a 

 quantity which can neither be increased nor diminished 

 by any mutual action of these bodies, though it may be 

 transformed into any one of the forms of which energy is 

 susceptible." 



An important addition was made to the knowledge of 

 energy and its manifestations when the true nature of 

 heat was discovered. By the old philosophers heat or 

 " caloric," as it was termed, was considered to be a 

 substance — an imponderable tluid, the addition of which 

 to a body made it hotter. Lord Bacon, however, seems 

 to have come to the conclusion that heat consisted of 

 a kind of motion or " brisk agitation " of the particles of 

 matter. In one passage he says : "It must not be 

 thought that heat generates motion, or motion heat — 

 though in some respects this is true — but the very essence 

 of heat, or the substantial self of heat, is motion and 

 nothing else." The true immaterial nature of heat could 

 not be more clearly stated than it is in these words, but 

 for long after Bacon's time the idea of the fluid " caloric " 

 was universal. 



About the end of last century Count Rumford published 

 an account of some experiments made by him. He had 

 been engaged superintending the manufacture of cannon 

 at the arsenal of Munich, and was surprised at the amount 

 of heat produced in the boring of the cannon. He made 

 measurements of the rise of temperature obtained in diffe- 

 rent cases, and having considered the various possible 

 sources of the heat developed, came to the conclusion 

 that it was in reality due to the friction of the boring 

 tool on the brass of the casting. "It is hardly necessary 

 to add," he says, " that anything which any insulated 

 body or system of bodies can continue to furnish withoul 

 limitdtiiin cannot possibly be a nuiterud suhxUinci' ; and it 

 appears to me to be extremely difficult, if not quite impos- 

 sible, to form any distinct idea of anything capable of 

 being excited and communicated in the manner that heat 

 was excited and communicated in these experiments except 

 it be imition." About the same time Sir Humphry Davy 

 melted two pieces of ice by rubbing them together, whilst 

 all surrounding bodies were kept at a temperature below 

 freezing-point. This experiment shows that heat cannot 

 be a substance, although Davy did not quite realise this 

 fact at the time. 



The most elaborate and important series of experiments 

 on the transformation of mechanical motion into heat were 

 those performed by .Joule near Manchester, between 1843 

 and 1K40. He caused paddles to rotate between fixed 

 vanes in a vessel of water, the motion of the paddles being 

 produced by the fall of a weight through a measured dis- 

 tance. The work done by the falling weight was 

 transformed through friction of the water into heat, 

 and a rise of tlie temperature of the water was pro- 

 duced. The distance through which the weight fell 

 being accurately observed, the mechrtiiical energy ex- 

 pended was known, and the amount of heat deve- 

 loped was shown by the rise of the temperature of the 

 quantity of water used through a definite number of de- 

 grees. By a large number of experiments of this sort. 



made with great precautions to ensure accuracy, -Joule de- 

 tei-mined that the amount of mechanical work represented 

 by the lifting of one pound through 772 feet would, if 

 transformed into heat, raise one pound of water 1° F. in 

 temperature. This amount of mechanical work — 772 

 foot-pounds, as it is called, taking as amount of work the 

 quantity required to raise one pound through one foot 

 against the attraction of gl•a^^ty — is called the rncihankal 

 cquiralent af hcnt. Another way of expressing the same 

 thing is to say that if a quantity of water faO through a 

 height of 772 feet, and tlaen be suddenly brought to a 

 standstill, its temperature vnW thereby be raised one 

 degree. This would be the case in a waterfall of that 

 height ; if the heat did not escape, the water below the 

 fall would be one degree hotter than that above. 



What does the heat consist of which can thus be pro- 

 duced by the transformation of the energy of a falling 

 body ? There can be little doubt that it consists of the 

 rapid motion or vibration of the minute particles of which 

 the body is composed. Our knowledge of these vibrations 

 can only be got at indirectly on account of the extreme 

 minuteness of the molecules of matter ; but it seems to 

 consist of a rapid backward and forward motion, somewhat 

 analogous to the motion of the particles of a sounding body. 

 Mechanical work, as we see in the case of a steam engine, 

 can be produced from heat. When this takes place it is 

 found that to produce a given quantity of work a certain 

 definite amount of heat must cease to exist. Under what- 

 ever conditions heat is changed into mechanical energy, it 

 is found that the heat which has passed out of existence is 

 equal in quantity to the amount which is mechanically 

 equivalent to the work generated. 



CAYENNE ECLIPSE EXPEDITION. 



THE photographs shown in the plates were taken 

 by Mr. Burnham in Cayenne, where he went to 

 observe the eclipse of December last. The upper 

 picture on the right-hand side is a photograph 

 taken in full sunlight in one of the streets of 

 Cayenne. The exposure must have been very short — 

 probably less than the yoo''1i °f ^ second, for the feet 

 of the woman descending from the kerb-stone behind, 

 and to tlie right of the child with the sunshade, appear 

 perfectly sharp when examined with a magnifier. The 

 centre of gravity of a person walking along at a rate of 

 three miles an hour moves through a little more than 

 .52 inches in a second, and the feet when steppmg forward 

 must move at at least double the rate of the body, or at 

 more than 100 inches in a second. A motion of an inch 

 in a direction at right angles to the line of sight during 

 the exposure would produce a recognisable blur ; but no 

 such blurring of the feet is recognisable in any of the 

 figures, thougli the feet are shown in all parts of the 

 step. 



The lower picture on the right hand was taken a few 

 minutes after the total phase was over. It represents the 

 observers roiuul the principal group of instruments, with 

 a few natives looking on. The negress to the right hand 

 wears the cliaracteristic Cayenne head-dress — a fiat board 

 covered with white cotton cloth. The observing-place was 

 within a French fort. It was not found necessary to 

 erect any tents or temporary wooden houses over the 

 instruments ; when left for the night, they were enveloped 

 in tarpaulins. 



The picture on the left shows a photographic telescope 

 equatorially mounted with driving-clock, and the weights 

 run down. Tlie photographs obtained during the eclipse 



