8 Prof. Leslie on Heat and Climate. [July, 



that quantity is greatly attenuated by the extent through which it 

 is spread. The accumulation of heat, therefore, in a given portion 

 of the mass, being on both accounts so slow, will soon be coun- 

 terbalanced by the dissipation that ensues. Several causes con- 

 cur to diminish the effect in water. The absorption of light is 

 extremely small in that fluid, and the heated portions dilating 

 rise continually to the surface and produce a general circulation ; 

 so that the heat is quickly diffused through the whole. If the 

 water exposes a surface equal to that of the lens or mirror, it 

 will be less heated than if set directly in the sun. 



It has often been objected to the theory of the identity of light 

 and heat, that the moon's rays collected in the focus of a power- 

 ful burning glass produce no sensible effect on the bulb of a 

 thermometer. But if we consider that her light is 300,000 times 

 weaker than that of the sun, we shall not be surprised that it has 

 not been detected by the rude experiments hitherto made. I 

 have no doubt, however, that the quantity might be ascertained, 

 if an exceeding nice thermometer were provided, capable of 

 bearing divisions to the -n^-th part of a degree, and the bidb 

 inclosed within a glass vessel from which the air is exhausted as 

 much as possible. 



It has been urged by some, that light and heat are distinct 

 fluids, but so related that the presence of the one occasions the 

 extrication of the other, because the former passes freely through 

 transparent substances, which the latter does not. Thus the 

 image of the fire in the focus of a lens produces no alteration 

 whatever on the thermometer, but in that of a concave mirror it 

 has a very sensible effect, which, however, would be prevented 

 by interposing a bit of glass. We must remark that the heat of 

 the fire is derived from two causes ; from the light emitted, and 

 from the warm air which flows on all sides from the vicinity of 

 the live coal. Bodies placed exti'emely near the fire are sensibly 

 affected by the light only, as appears from an experiment of my 

 worthy friend Mr. T. Wedgwood ; but at a considerable distance 

 the influence of the light may be totally disregarded . It is easy 

 to see that a concave mirror will throw back the streams of heated 

 air towards its focus, while a lens will completely stop their 

 progress. For the same reason, hardness and polish are not 

 necessary to the reflector, and a wooden bowl will produce the 

 same effect. Even a slender netting composed of numerous 

 meshes will prevent in a great measure the communication of 

 heat by opposing a considerable resistance to the passage of the 

 warm air; and, from the same principle, if a circular hole, not 

 exceeding an inch in diameter, be cut in a fire-screen, a thermo- 

 meter placed a little behind the aperture will scarcely be affected. 

 If a hot body be placed in one of the conjugate foci of two con- 

 cave mirrors, the air which flows from it upon the nearer mirror 

 will be thrown towards the more remote, and thence reflected to 



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