HEAT AND COLD. 



since the unequal additions of heat given to the water and the 

 mercury produce precisely the same effects upon it. But even 

 though we omit the consideration of the relative quantities of 

 heat that produce equal changes of temperature in different 

 bodies, the sense of feeling will still be found most fallacious in 

 the indications which it gives of temperature itself ; and here, 

 indeed, the error and confusion into which it is apt to lead, 

 when unaided by the results of science, are very conspicuous. 

 The air of a cave, if it be sufficiently deep, will feel cold in 

 summer, and warm in winter. If a thermometer be suspended 

 in it, it will prove that its temperature is always the same. In 

 summer, that temperature being below that of the general 

 atmosphere, the cave feels cold ; in winter, being above it, the 

 cave feels warm. The same thermometer which has been kept 

 for sixty years in the vaults of the Observatory at Paris, at the 

 depth of eighty-eight feet below the surface, has shown, during 

 that interval, the temperature of 11 -82 Cent., which is equal to 

 53 ;p Fahr., without varying more than half a degree of Fahr., 

 and even this variation, small as it is, has been explained by the 

 effects of currents of air produced by the quarrying operations in 

 the neighbourhood of the Observatory. 



It appears, therefore, that our perception of heat or cold 

 depends not alone on the thermal state of the bodies which affect 

 us, but also on the state of our own bodies at the moment. These 

 perceptions are, in effect, relative, and not absolute. One body 

 feels cold because it is below, and another warm because it is 

 above, the temperature of our own bodies. 



It follows, therefore, that if we reduce, by any expedient, 

 different members of our bodies to different states of warmth, 

 n,ny external object which has one intermediate temperature 

 will feel warm to the colder, and cold to the warmer member. 

 This experiment may be easily tried. If we hold the hand in 

 water which has a temperature of about 90, after the agitation 

 of the liquid has ceased, we shall become wholly insensible of 

 its presence, and shall be unconscious that the hand is in contact 

 with any body whatever. "We shall, of course, be altogether 

 unconscious of the temperature of the water. Having held both 

 hands in this water, let us now remove the one to water at a 

 temperature of 200, and the other to water at the temperature 

 of 32. After holding the hands for some time in this manner, 

 let them be both removed, and again immersed in the water at 

 90 ; immediately we shall become sensible of warmth in the one 

 hand, and cold in the other. To the hand which had been 

 immersed in the cold water, the water at 90 will feel hot, and 

 to the hand which had been immersed in the water at 200, the 



91 



