392 Royal Irish Academy : Dr. Apjohn on the 



tenth of an inch in thickness. The stop-cock was now closed, and 

 the apparatus being lowered, the whole was left to itself until the 

 following day, when the first of a series of observations, continued 

 for twenty successive days, was made, each comprehending the vo- 

 lume of the moist air, the pressure, and the temperature both of the 

 air and of the mercury in the barometer. To deduce from these by 



the formula / = 



X p, the force of vapour, it was necessary, 



in the first instance, to apply to p all the corrections already ex- 

 plained, and in addition to raise 91T64, the volume of the dry air, 

 to what it would be at the temperature and pressure of the moist 

 air, as noted in each observation. But, as this involved tedious 

 arithmetical computations, and as the thermometer during the per- 

 formance of the twenty experiments varied only about 15°, I came 

 to the resolution, being at the time upon the eve of leaving town for 

 a couple of months, to postpone the calculations until I should be 

 possessed of data applicable to the solution of the problem I had un- 

 dertaken, throughout a more extended range of temperature. 



Accordingly, in November last, I resumed the subject with the 

 very same apparatus, which had been left statu quo in the interval, 

 and succeeded in completing a series of forty-five additional observa- 

 tions, extending nearly as low as 32°, and which I had every reason 

 to expect would lead to satisfactory results. Upon, however, sub- 

 mitting the whole to calculation, I have been led to the mortifying 

 conviction, that in consequence either of the absorption of the oxygen 

 by the mercury and brass-work, or some accident which befel the 

 apparatus during my absence from town, the entire of the latter 

 series of observations is of no value, as they lead to results for the 

 force of aqueous vapour, which are certainly greatly below the truth. 

 Upon the present occasion, therefore, I can direct attention only to 

 the observations made in July and August last. These are contained 

 in the following table, and, as has been already stated, they amount 

 to twenty in number, the highest temperature having been 65°, and 

 the lowest 49 0, 6. The numbers in the last column represent the 

 bulks which the 911*64 volumes of dry air would have, if reduced to 

 the temperature t, and the corrected pressure p. 



Table I. 



