Royal Society, 565 



the air, the temperature of evaporation, and the mean depression of 

 the wet- bulb, the means of these elements have been taken and are 

 presented in a table. In this table are also given the dew-points as 

 determined by means of Mr. Glaisher s factors and by Dr. Apjohn's 

 formula, with the differences by the two methods. The author re- 

 marks that the first feature which presents itself, in running the eye 

 over this table, is the enormous amount of the depression of the wet- 

 bulb compared with our European experience. In March, the mean 

 depression at no hour was less than 14?°*8 at 7 a.m., increasing to 

 29°*6 at 3 P.M. ; in April, the mean depression was never less than 

 17°'3 at 7 A.M., increasing to 29°*9 at 3 p.m. ; and many observations 

 necessarily much exceeded the maxima means. The next feature is 

 the increase of the mean depression with that of the mean tempera- 

 ture, from 6 a.m. until 3 p.m., and then a decline with the decline of 

 temperature until 9 p.m. ; but not in the same ratio as the increase 

 in the morning. With reference to the practical application of these 

 observations with a view to determine the amount of moisture in the 

 atmosphere, or to fix the dew-point, the author remarks that it will 

 be seen from this table that Mr. Glaisher's factors give a higher dew- 

 point than Dr. Apjohn's formula, varying in March from 6°'l at 8 a.m. 

 to 1 1°-9 at 6 p.m., and in April from 5°'6 at 7 a.m. to 10°*4 at 9 p.m. ; 

 and these varying discrepancies do not appear to have gradations of 

 increment or decrement dependent upon increase or diminution of 

 mean temperature, or increase or decrease of the depression of the 

 wet-bulb. These remarks apply to the means of the observations ; 

 but with reference to isolated observations, the discrepancies by the 

 two methods become much greater. On the 9th of April, at 8 p.m., 

 the temperature of the air being 97°, the wet-bulb with a moderate 

 draught 60°*5, and with a strong draught 60°, the depressions were 

 respectively 36°*5 and 37°, and the dew-point for the latter depres- 

 sion determined by Mr. Glaisher's factors would be 41*5, and 12*6 

 by Dr. Apjohn's formula. In illustration of this part of the subject 

 the author gives an extract of a letter from General CuUen, from 

 which it appears that at Cochin on the Malabar coast, the tempera- 

 ture of the air being 96°, the wet- bulb 61°, the dew-point by Jones's 

 hygrometer 38°, the dew-point by Mr. Glaisher's factors would be 

 43°-5, and by Dr. Apjohn's formula 22°-l. 



" On a General Law of Density in saturated Vapours." By 

 J. J. Waterston, Esq. Communicated by Lieut.-Colonel Sabine, R.A., 

 V.P.andTreas.R.S.&c. 



The author of this paper commences by stating that the relation 

 between the pre ssure and temperature of vapours in contact with 

 their generating liquids has been expressed by a variety of empirical 

 formula?, which, although convenient for practical purposes, do not 

 claim to represent any general law ; and that some years ago, while 

 examining a mathematical theory of gases, he endeavoured to find 

 out, from the experiments of the French Academy, whether the den- 

 sity of steam in contact with water followed any distinct law with 

 reference to the temperature measured from the zero of gaseous ten- 

 sion (situated at — 4<61°Fahr. by Rudberg's experiments, confirmed 



