268 Prof. Apjohn on a Formula for ascertaining 



The numbers in column (6.) are the dew-points calculated 

 by my formula ; and while there is an almost exact corre- 

 spondence between the first and the result of experiment, the 

 second, it will be seen, is higher than the observed tempera- 

 ture of deposition by nearly three degrees. There is here, 

 however, obviously some mistake. It is impossible that with 

 the recorded temperatures of air and hygrometer the dew- 

 point could have been so low ; and this conclusion I do not 

 at present draw from my theoretical views, for that would be 

 to subject myself to the imputation of arguing in a circle, but 

 from the following observation made by me with great care 

 on the 22nd of March. 



Here the temperatures t and t f differ from those taken from 

 the Encyclopaedia only by about half a degree, and neverthe- 

 less the observed dew-point 44 is higher than 39*5 by 4*5 de- 

 grees. From these observations, therefore, I am, I conceive, 

 entitled to conclude, 1st, that the series in which the depres- 

 sion amounts to 15 0, 2, being in exact accordance with my 

 formula, lends it some degree of support ; and 2ndly, that my 

 method cannot be considered as impugned by the other 

 series, in as much as this is in some particular manifestly incor- 

 rect. But it is time to enter upon the experimental tests to 

 which I have resorted. 



If air, in reference to which t 9 t\ and t !l have been accu- 

 rately noted, be raised to any elevated temperature, and the 

 observation be repeated in the heated air as far as respects 

 /and /', we shall have two* separate sets of observations, from 

 which to calculate the point of deposition ; and as the amount 

 of moisture in the air is not altered by the augmentation of 

 temperature it has experienced, both calculations, provided 

 our formula be correct, should give precisely the same result, 

 2. e. the dew-point in the first instance determined by observa- 



* Any number of observations, having reference to the same dew-point, 

 may, it is obvious, be thus obtained. 



