526 Mr. Pembrey on Comparative Experiments 



chemical method. The mean difference, however, in the 

 above series was insensible*. 



The Chemical Hygrometer employed in both series was 

 that introduced by Dr. Haldane and the author f. 



The absorption-tubes were placed in a small wooden box 

 with wire partitions to prevent them from knocking against 

 each other. The entrance-tube, by which the air to be 

 examined passed into the hygrometer, was fixed through a 

 small perforation in a rubber partition covering a hole in 

 the box. In this way any possibility of air being taken from 

 the inside of the box was avoided. 



The comparative experiments were made in the following 

 manner. The weighed absorption-tubes were placed in the 

 shed containing the psychrometer about ten inches below the 

 bulbs of the thermometers. The wet- and dry-bulbs were 

 then read off ; the absorption-tubes were connected by a long 

 piece of rubber-tubing with the aspirator. Air was now 

 drawn through the tubes at a rate of 1500 cub. centim. per 

 minute, until about 11,500 cub. centim. of the air had been 

 taken. Five readings of the temperature of the water and of 

 the air in the aspirator were taken during each period of 

 observation. When the aspiration was finished, the readings 

 of the wet- and dry-bulbs were again taken, and the absorption- 

 tubes disconnected and stoppered. The period of observation 

 generally lasted about ten minutes. 



Simultaneous determinations with two chemical hygro- 

 meters were made in the previous, but not in the present 

 series of observations. In order, however, to check the com- 

 pleteness of the absorption and any errors in weighing, a 

 second pair of absorption-tubes was connected up with the 

 first pair. 



The results for the psychrometer were calculated, not from 

 my own readings, but from the mean of three other readings 

 ■ — one at the beginning, one at the middle, and one at the end 

 of each period of aspiration. These readings were obtained 

 from the continuous photographic record of the wet- and dry- 

 bulbs taken at the Radcliffe Observatory. It is possible to 

 read off this record to two minutes and to one-tenth of a 

 degree Fahrenheit. The accuracy of the readings has been 

 proved by years of use and comparison with eye-readings. 



In every case care was taken to have the wet-bulb properly 

 moistened about a quarter of an hour before the observations. 



* Phil. Mag. April 1890, p. 314. 



t " An Improved Method of Determining Moisture and Carbonic Acid 

 in Air." Haldane and Pembrey, Phil. Mag. April 1890. 



