51 H. F. Blanford — Experiments with the Psychrometer. [No. 2, 



more below the temperature of the air, and observations made at a distance 

 from all the convenient appliances of a physical laboratory, in hot winds 

 and under the intense glare of air Indian sun, are not easily endowed with 

 that precision which is desirable and which is easily to be attained in a well 

 fitted observatory of a high class. I can only say that I have endeavoured 

 to take all such precautions as were practicable and if the range of indivi- 

 dual error is on the whole considerable, I believe that the mean result 

 affords a trustworthy criterion of the comparative value of the psychrome- 

 tric methods ; and that, with proper precautions, very trustworthy data of 

 the hygrometric state of the atmosphere may be obtained with the dry and 

 wet bulb thermometers, at all events when the humidity does not fall below 

 20 per cent, of satiu*ation. Greater degrees of dryness I have as yet had 

 no opportunity of testing ; though such are prevalent somewhat later in the 

 season than when my observations were made ; especially in Sind and the 

 Punjab, and on the plateau of Central India. 



The earlier series of observations were made during an inspection tour 

 in the Madras Presidency, in April 1875. I regret that the original rough 

 record has been mislaid, and I am able to give only the means of each set 

 of readings. In these series (Nos. 1 to 13 of the Tables), the means adop- 

 ted for the readings of the Regnault's hygrometer include the temperatures 

 at which the dew disappeared from the silver capsule of the instrument, 

 which is not the case with the later series. In striking the adopted mean 

 of each set of readings, the mean temperature at which dew was deposited 

 and that at which it disappeared were taken separately, and the mean of the 

 two results adopted as the dew-point. The difference, however, rarely 

 amounted to a degree, and is very small in comparison with the difference 

 of the temperature and that of the dew-point. 



The later series were made during a recent tour in Upper India, in the 

 months of March and April. The air temperatures are generally lower than 

 the Madras series, but the dew-points are proportionally lower. They 

 indicate a very dry atmosphere, although not so low a relative humidity as is 

 shewn later in the season by the registers of many stations in the interior. 



The same hygrometer has been used throughout. It is one of Casella's 

 manufacture, and is of the form represented in his illustrated catalogue ; 

 it has a single capsule, the air-thermometer being freely exposed ; and the 

 evaporation of the ether is accelerated by blowing from the mouth through 

 a piece of elastic tubing about 15 inches in length. Both the thermome- 

 ters have been verified by myself ; at the freezing point by immersion in 

 crushed ice ; and through the range of observation, by comparison with a Kew 

 standard (No. 374) which I received in 1868 from Prof. Balfour Stewart. 



At the Madras stations (excepting Trichinopoly and Madras), the 

 psychrometric observations were made with the observatory thermometers 



