Prof. Apjohn on ascertaining the Dew-point* 267 



in which f" is the tension of steam at the dew-point, /' its 

 tension at the temperature of the hygrometer, d the de- 

 pression or difference between the temperature of the hygro- 

 meter and air, p the existing and 30 the mean pressure, and 

 m a coefficient depending upon the specific heat of air and 

 the caloric of elasticity of its included vapour, its arithmetical 

 value being *01 149, or the equivalent vulgar fraction ^ 7 . ^ n 

 the paper to which I have already referred corrections are 

 given for the influence on the specific heat of air of the fluc- 

 tuations of the barometer, and the moisture present in the 

 atmosphere. These corrections are, I believe, deduced from 

 correct principles, and should be resorted to when extreme 

 precision is desirable. Experience, however, and a careful 

 consideration of the subject have satisfied me that they are, 

 generally speaking, in their effects much too insignificant to be 

 objects of attention to the practical meteorologist. 



The first and most obvious method of verification which 

 presented itself to my mind was the comparison of my formula 

 with recorded cotemporaneous observations on the tempera- 

 ture of air, that shown by a moist-bulb hygrometer, and. the 

 actual dew-point. I have, however, unfortunately been able to 

 meet but few at all suited to my purpose. Those in which 

 t — t' is small, and this is generally the case in the few re- 

 gisters to which I have had access, cannot, as we have already 

 seen, serve for deciding the value of any formula. In the First 

 Report, indeed, of the British Association for the Promotion of 

 Science, page 50, mention is made of a register of observations 

 kept in the East Indies, which, as belonging to high tempera- 

 tures, would necessarily exhibit great depressions, and would 

 therefore be valuable as a standard of comparison ; but 1 have 

 in vain searched for the Calcutta Journal " Gleanings in 

 Science," in which they are said to be contained. In fact, the 

 only observations I have been able to procure adapted to my 

 purpose, and made, apparently at least, with the necessary 

 precision, are those adduced in the article Hygrometry of 

 Sir David Brewster's Encyclopaedia, and there made by the 

 author of the article the basis of a calculation for investigating 

 the constants of a tentative formula for connecting the indi- 

 cations of the wet-bulb hygrometer with the dew-point. They 

 are but two in number, and are comprehended in the follow- 

 ing table, in which the numbers in the first column represent 

 the temperatures of air, those in the second the correspond- 

 ing indications of the hygrometer, those in the third the de- 

 pressions, those in the fourth the pressures, and those in the 

 fifth the dew-points experimentally determined by the method 

 of Dal ton. 



2 M 2 



