SENSE OF TEMPERATURE. ANTENNA OF INSECTS. 393 



without the other being affected. It is rather of a comparative 

 than of a positive kind ; that is, we form our estimate of tem- 

 perature rather by comparing it with that to which our body 

 (or the part of it employed to test the heat or cold) has been 

 previously exposed, than by any knowledge which we derive 

 through the sensation as to the actual degree of heat or cold 

 to which the organ is exposed. Thus, if we plunge one hand 

 into a basin of hot water, and the other into cold, and then 

 transfer both of, them to a basin of tepid water, this will feel 

 cold to the hand which has been previously accustomed to 

 the heat, and warm to the other. In the same manner, the 

 temperature of Quito, which is situated half-way up a lofty 

 mountain, is felt to be chilly by a person who has ascended 

 from the burning plains at its base, whilst it seems intensely 

 hot to another who has descended from its snow-capped sum- 

 mit ; the residents in the town at the same time regarding it 

 as moderate, neither hot nor cold. It is a curious circum- 

 stance, that a weak impression made on a large surface seems 

 more powerful than a stronger impression made on a small 

 surface ; thus, if the fore-finger of one hand be immersed in 

 water at 104, and the whole of the other hand be plunged in 

 water at 102, the cooler water will be thought the warmer ; 

 whence the well-known fact, that water in which a finger can 

 be held will scald the whole hand. 



498. Where any special organs of Touch exist in Inverte- 

 brated Animals, they are for 

 the most part prolongations 

 from the portion of the head 

 near the mouth. This is 

 the case with the arms of 

 the Cuttle-fish, and with the 

 tentacula of the lower Mol- 

 lusca which are similar in 

 position. Among Crustacea 

 and Insects, the antennae or 

 feelers (fig. 198, a, a) appear 

 to be the special organs of 

 touch. These are frequently Fig - ^.-CAPRICORN-BEETLE. 

 very long, and present an extraordinary variety in their forms, 

 of which some are depicted in fig. 199. They contain, 

 for the most part, a large number of joints (in the Mole- 



