so Mr Henry Meikle'*s Remarks and Eocperiments 



Expansions * ; and I have some suspicion, that, in their very 

 elaborate experiments on the coolingof large thermometers, they 

 have overlooked the influence of change of pressure ; the effects 

 of which were the more to be feared, on account of the gigantic 

 size of the bulbs, and the great range through which they ope- 

 rated. The glass of large thermometers is usually thinner, es- 

 pecially in proportion to their diameters^ than of small ones ; and 

 if it was so in their case, the errors would be so much the greater ; 

 but these learned authors have given us no data from which the 

 amount of such an effect could be estimated. This, however, 

 they might still do, if the instruments be preserved. 



Farther experiments are perhaps wanted, regarding the depres- 

 sion of wet thermometers ; but at present, I may mention that 

 the two sets which I have given above, especially the firsts make 

 the depression, through a range which will seldom be exceeded, 



nearly proportional to ^ , where B is the height of the baro- 

 meter in inches ; and probably a still more exact number might 

 be found, by which the observed depression being divided, will 

 be reduced to what it would have been under the standard pres- 

 sure. As a temperature of 60° rarely occurs at great elevations, 

 the last table is not suited to their case ; and, therefore, its de- 

 viating a little from this formula, when the pressure is small, be- 

 comes a matter of no moment. From these experiments, it ap- 

 pears, that the variations of pressure have much less influence 

 on evaporation than is commonly supposed ; and that, on the 

 same spot, variations of atmospheric pressure may, without much 

 danger of error, be neglected. 



According to Professor Leslie and Mr Ivory, § the depres- 

 sion of the moistened thermometer, under the same pressure, is 

 proportional to the drying quality of the air after its temperature 

 is so reduced. Or, a given volume at that reduced temperature, 

 can still retain c ^ more grains of moisture than is already con- 

 tained in the like bulk of surrounding air ; c being a constant 

 coefficient to be determined by experiment. Hence, if w =: ac- 

 tual weight of moisture in the given volume, at the existing tem- 



• Annales de Chim. et de Phys. tome vii. ; Anuals Phil. xiii. ; and for Mr 

 Crichton's remarks, see Annals Phil, xxiii. 



