Page:Journal of the American Society of Mechanical Engineers, Volume 33.pdf/662

than given by their line. If their values were accepted at atmospheric temperatures we would have a specific heat for air considerably lower than that given by Regnault, while psychrometric evidence seems to indicate that it should be considerably higher at such temperatures. The most reliable of recent determinations would seem to be that of W. F. G. Swann.^[1] In his paper he points out a defect in the method of Regnault which would account for the latter’s value being too low. The values given by Swann have therefore been adopted in this paper; although they appear still to require confirmation, since there would seem to be considerable opportunity for error in the method of air measurement used in his experiments.

44The equation given for the specific heat of steam at low temperatures seems to agree fairly well with modern experimental data. Extreme accuracy is not pretended, nor is this essential at lower temperatures, since under 150 deg. the total heat value of the air is affected only 2 per cent at most by the specific heat of the water vapor. The values given by this equation, however, are undoubtedly more nearly correct at lower temperatures than the usual value, $C=0.48$.

45The psychrometric charts, Figs. 1 and 2, are constructed accurately from the foregoing data. Fig. 2 exhibits all psychrometric relationships, between the temperatures of 20 deg. and 350 deg. and saturation temperatures up to 143 deg. Fig. 1 gives the same values between temperatures 20 deg. and 110 deg. and saturation temperatures to 95 deg. These charts are here shown to a greatly reduced scale. In its original form, Fig. 1 permits the reading of both the wet and dry-bulb temperatures to an accuracy of 0.1 deg. and of the moisture weight per pound of air to 0.2 grains. All calculations have been made with accuracy to five significant figures by means of a Thatcher slide rule.

SATURATION CURVE

46 The saturation curve, Fig. 2, expressed in grains of moisture per pound of air, was computed from the formula

[5]

$G={\frac {5284(t+459.64)D_{s}}{P-e}}$

where

$G$ = grains of moisture per lb. of pure air at saturation

$t$ = temperature of saturation in deg. fahr.

1. Phil.Trans.Royal.Soc., series A, vol. 210, pp. 199-238, 1909.