Page:Journal of the American Society of Mechanical Engineers, Volume 33.pdf/653

heat received by radiation becomes a small percentage of the total heat transformation.

23⁠The most reliable tables based on the stationary wet-bulb hygrometer are those by James Glaisher (1847)^[1]. The tables of the United States Weather Bureau based upon an empirical formula deduced by Prof. Wm. Ferrel from simultaneous determinations with the sling psychrometer and the dew-point instrument are more reliable, and are now generally used. The limitations of this formula are admitted, since it is held to be correct only over the range of observation from which it was deduced, including simply temperatures below 120 deg. fahr.

24⁠Professor Ferrel’s formula as given in the tables of the United States Weather Bureau is

$e=e'-\left(0.000367P\right)\left(t-t'\right)\left(1+{\frac {t'-32}{1571}}\right)$

where

$e$ = partial pressure of the moisture in the air, which also = vapor pressure corresponding to the dew point

$e'$ = the vapor pressure corresponding to saturation at wet bulb temperature $t'$

$P$ = the barometric pressure

$t$ = dry-bulb temperature in deg. fahr.

$t'$ = wet-bulb temperature in deg. fahr.

25⁠The temperature of the dew point is found by selecting the temperature corresponding to the pressure $e$, from the temperature pressure diagram or table. The per cent of relative humidity is $R={\frac {e}{e_{t}}}$, where $e_{t}$, is the vapor pressure corresponding to the dry-bulb temperature $t$, as previously demonstrated. The absolute humidity expressed in grains of moisture per cubic foot is then determined by multiplying the grains of moisture per cubic foot corresponding to saturation at dry-bulb temperature by the per cent of relative humid ity thus determined.

26⁠The writer would substitute for such an empirical formula a rational one, having a thermodynamic basis, that is, a formula depending upon the transformation of sensible heat into latent heat in the adiabatic saturation of dry air.

27⁠Historically, it is of interest to note in this connection, that James Apjohn^[2] propounded in 1836 this same theory of wet-bulb

1. Phil. Trans. Royal Soc., 1851, p. 141.
2. Irish Academy Trans., vol. 17, pp. 275-282, 1837.