372 S. A. Hill — Psychrometer and Condensing Hygrometer. [Ko. 4, 



have been corrected for scale error, as determined by comparison in 

 water with a Kew standard. For the second series, these errors were 

 re-determined, and the standard thermometer verified at the freezing 

 point, at the beginning of April. The dew point thermometer is one of 

 Casella's with a cylindrical bulb and a long scale which may be read 

 ofl: with ease to the tenth of a degree from a distance of three or four 

 feet. The dry and wet bulb instruments are by Hicks. They have 

 spherical bulbs about | of an inch in diameter and they have been very 

 carefully graduated, their corrections to the standard at the present time 

 being as follows : — 



No. 7 (Dry) No. 8 (Wet) 



Below 85°, — 0*7° Below 57°, — Q-T 



Above 85°, — 0-8° Above 57°, — 0-8° 



The condensing hygrometer is much more sensitive, or responds 

 much more readily, to variations in the dew point than does the wet 

 bulb thermometer. This may be partly the effect of its thermometer 

 having a large cylindrical bulb and a capillary tube. Thus, when both 

 instruments were exposed to a hot wind on the afternoon of the 7th 

 April at Allahabad, the following fluctuations of the dew point were 

 observed in a period of less than three minutes : 36-9°, 35*9°, 33 7°, 

 34!'5°, 30*7°, 34'4°. During this time the dry thermometer varied from 

 104-2° to 105-2° and back again to 104-3°, whilst the wet bulb stood 

 constant at 66-7°. 



Every dew point observation in the table, except No. 4 of Series II, 

 represents the mean of at least four separate observations, made alter- 

 nately at the moments of appearance and disappearance of dew. Every 

 entry under dry bulb and wet bulb temperatures is the mean of two 

 observations made immediately before and directly after the correspond- 

 ing dew point observations, and the time to which they are referred is 

 approximately the mean time of the whole set of observations. 



Except in the Allahabad observations, in which I was assisted by 

 Pandit Soti Raghubans Lai, a student in the M. A. class of the Muir 

 College, all the observations have been made by myself. The second 

 series has been expressly designed to determine if possible the influence 

 of various degrees of ventilation upon the indications of the psychro- 

 meter. 



