Page:Journal of the American Society of Mechanical Engineers, Volume 33.pdf/655

1322 31from these three fundamental principles there may be deduced a fourth:

32A statement of the experimental demonstration of these four principles is given in Appendix No. 2.

33In consequence of the psychrometric principles A, B and C, the moisture content of air from accurate psychrometric readings may be easily computed, provided the required temperature is known, as well as the density relations in a mixture of pure air and saturated water vapor, and also the exact latent heat of water vapor, and the speciﬁc heat of air and of water vapor at any temperature.

34No novelty is claimed for this method since the writer found, while preparing this paper, that this very means had been proposed by James Apjohn. However, it does not seem to have been taken very seriously by contemporary scientists since it was never properly developed. Moreover, the details of his method were such as to make it worthless.

35Recent research into the properties of water vapor has fully established its properties to a great degree of exactness, with the possible exception of the speciﬁc heat, which is of minor importance in psychrometric calculations. The author, however, was surprised to ﬁnd upon investigation that the usual value assigned to the speciﬁc heat of air was unquestionably incorrect, since it had been deﬁnitely proved to be variable, and not a constant as assumed by Regnault. Moreover, recent investigators conducting their experiments with modern apparatus, supposedly with extreme accuracy, differed from each other by more than 3 per cent, and from the generally accepted value of Regnault, by more than 2 per cent. Therefore, in order to use a rational formula in the construction of accurate psychrometric charts and tables, it becomes necessary to determine the speciﬁc heat of air to a much greater degree of accuracy than is known at present.