The Cooling and Evaporative Powers of the Atmosphere. 447 



To lest the accuracy of this formula, experiments were carried out iu still 

 air under conditions of increased and diniiuislied pressure. In the former 

 case the value of the pressure was obtained by a gauge reading in pounds, a.nd 

 was only approximate ; in the case of diminished pressure a mercury gauge 

 was used reading to tenths of a centimetre. The results obtained are shown 

 in the Table given below. To allow for the variation of temperature which 

 took place, each of the cooling powers Hi is reduced to that cooling power 

 H2 which it would have been if the temperature had been 11"7° C, this 

 being the temperature at which the normal pressure (764 mm. or 15 lbs.) 

 reading was taken ; the formula used to obtain H2 being 



The column Hii^alculated) gives the values obtained by equation (viii) 

 Hi being found experimentally to be 6'94 at temperature 11-7° and pressure 

 pi 764 mm. 



The agreement between the experimental and calculated values are close, 

 and this agreement seems to confirm the experimental observation that half 

 the rate of heat loss at ordinary temperature is due to radiation. 



Cooling of Kata in Compressed and Earefied Air. 



Mean time of 

 cooling in seconds. 



Mean 

 temperature. 



Pressure. 





Ho 



(experimental). 



(calculated). 



61 -7 



11 -8 



29 lbs. 



8 -10 



8 -15 



8-29 



63-8 



10-8 



22 -5 „ 



7-84 



7-57 



7-74 



72 -1 



11-7 



764 mm. 



6-94 



6-94 



6 -94 



81 -1 



12-7 



572 „ 



6-17 



H -42 



6-47 



S8-3 



12 -9 



425 „ 



5-66 



5 -95 



6-05 



95-1 



13-4 



335 „ 



5-26 



5 '65 



5 "77 



VOL. XC. — B. 



