HEAT, COLD, AND LIGHT. 313 



corresponding divisions : it is frequently found 

 that the tubes, instead of being cylindrical, are 

 smallest in the middle, and widen gradually to- 

 wards each end, like two conic frustrums joined 

 at their less bases. When this is the case, it is 

 proper that the divisions, instead of being equal, 

 should be longest in the middle, and decrease by 

 regular gradations toward each end, so that the 

 cavity in the tube shall contain the same quantity 

 of fluid between every two division!. 



We now proceed to the more immediate objects 

 of this Appendix, namely, Heat, Cold, and Light: 

 The two latter of these will not be considered very 

 attentively; only as they are connected with the 

 former, which will be handled more at large. 



Heat and Cold are to be considered either as 

 particular sensations, or as the causes of powers 

 which bodies possess of exciting those sensations. 

 Thus, we say that we ourselves are hot or cold, 

 and that the fire or ice which heats or cools us is 

 likewise hot or cold : though the sensations we 

 experience are certainly very different things from 

 that which enables those bodies to excite them. 

 It must be observed that the sensations of Heat 

 and Cold are very fallacious ones, in so far as 

 they are effected by the temperature of the body 

 in which they are excited : for we may feel a sub- 

 stance hot when it is in the same circumstances in 

 which we should feel it cool at another time. To 

 elucidate this, we may observe that when we have 

 been accustomed to live in an atmosphere of be- 

 twixt 4- 60? and + 70, if the Heat fall to + 60? 

 p \vc 



