136 MR. W. N. SHAW ON HYGROMETRIC METHODS. 



The results show that AUGUST'S formula gives the dew-point from psychrometer 

 observations with considerable accuracy, even when the dew-point is very far below 

 the temperature of the air. The further conclusion to be drawn from the observa- 

 tions is already made clear, namely, the necessity for the exposure of the wet-bulb to 

 a sufficient current of air. The readings are too high by all the computations when 

 the exposure was in the verandah or room. 



BLANFORD does not regard his dew-point observations as unexceptionable, and calls 

 attention, among other practical points in connexion with this observation, to the 

 necessity for a highly burnished surface for the thimble and freedom from microscopic 

 scratches, as these make it very difficult to seize the moment of definition when the 

 humidity is very small and amount of deposit consequently light. 



The attempts at deducing theoretically a formula for the computation of psychro- 

 metric observations, to which allusion has hitherto been made, have been based upon 

 the assumption that there was a continuous replacement of a layer of air, no matter 

 how thin, surrounding the wet bulb. This layer of air was supposed to be reduced to 

 the temperature of the wet bulb, and completely saturated with moisture. The 

 effect of the varying rapidity of motion of the air was not taken into account, and 

 the effect of radiation was regarded as insensible. An equation is obtained by 

 AUGUST, APJOHN, and others, between the amount of heat supplied to the surrounding 

 air and the amount lost by the evaporating liquid. BELLI, on the other hand, 

 equates the mass of air reduced, per unit time, from the temperature, t, of the dry to 

 the temperature, t', of the wet bulb, to the mass of air saturated by evaporation per 

 unit time. This slight divergence between the methods of obtaining the equation 

 leads to no difference in the results, because the theory evidently represents ultimately 

 the same physical state of things. The matter is, however, treated from a different 

 point of view in MAXWELL'S article on " Diffusion " in the ' Encycl. Britann.' (9th 

 edit.). In the discussion of the physical problem, as there treated, he considers a 

 " steady " distribution of moistui-e and temperature round the wet bulb, supposed to 

 be maintained in a perfectly still atmosphere of indefinite extent, the distribution of 

 heat and moisture being brought about by conduction, radiation, and diffusion. 

 MAXWELL calls this the conduction and diffusion theory in calm air, in contradistinc- 

 tion to the convection theory referred to above. The mathematical solution is 

 identical in form with that of the corresponding electrical problem to find the distri- 

 bution of potential, due to a charged body in a field, and consequently, with the 

 correction for radiation, leads to the following equation : 



PS [K All 



^^-^{D + I^S 



where 



PS is the vapour pressure in the air undisturbed by the presence of the 



wet thermometer bulb. 

 p l is the vapour pressure at the surface of the bulb. 



