114 



MR. W. N. SHAW ON HTGROMETRIC METHODS. 



We may perhaps disregard WILSON'S hygrometer, which is a mercury thermometer, 

 with a rat's bladder bulb. A good deal of the article is taken up with a discussion 

 of some of the therraodynamic properties of air, not much to the point. 



We have next a discussion of dew-point methods, and a description of DANIELL'S 

 hygrometer, and an exposition of its defects, as well as those of the various sugges- 

 tions made as improvements on DANIELL'S, of which there are many. 



A table of results of comparisons of DANIELL'S and various instruments of this 

 kind is given, and the mean errors (28 experiments) from the results obtained by 

 LEROY'S method (cooled water in an open vessel) are given as follows : 



A DIE'S . . , 

 DANIELL'S . . 

 Spherical bulb 

 Long bulb . 



- O'l 

 + 2-9 



- 478 



- 6-6 



The author of the article then turns his attention to the wet-and-dry-bulb method, 

 which he says has been sadly neglected for 30 years. The following extract will 

 show the application of a formula of reduction to very wide variations in the condi- 

 tions of observation ; the result is very fairly satisfactory : 



" The results of experiments determining the dew-point for a considerable number 

 of indications of the wet and dry thermometers, and under various pressures, though 

 principally at pretty high temperatures, are given in a Calcutta journal, ' Gleanings 

 in Science,' Nos. 2 and 3, 1829, and in the 'Edinburgh Phil. Journ.' for October, 

 1883, from which we have obtained the following table. The sixth column is derived 



from the formula 



(/, + -66372) (t- f) _ f 

 1 75-438 f t 



where t is the Fahrenheit temperature of the air, t' that of the moist bulb, and t" the 

 dew-point ; andyj, f t ,, f t ,, are the forces of aqueous vapour in a state of saturation at 

 these temperatures respectively. 



