Psychroinetric Formula. 211 



dew-point thermometer being practically zero throughout. 

 Nos. 2 and 3 were in agreement over the greater part of the 

 scale. All temperatures were recorded to the nearest twentieth 

 of a degree ; owing to the smallness of the spaces between the 

 graduations and the unavoidable slight fluctuations during the 

 course of an experiment, any nearer approximation gives only 

 an apparent increase of accuracy. Moreover, tables of vapour- 

 pressure at present differ from one another by amounts com- 

 parable even to the tenth of a degree. The table actually 

 employed was that obtained by Broch by reduction of Reg- 

 nault's observations, as given .in the Physical Tables, Smith- 

 sonian Miscellaneous Collections, vol. 35. 



Each observation involved the following procedure : — The 

 three wet-bulb thermometers were suspended with their coat- 

 ings dipping into water, and left to attain equilibrium w4th 

 the surrounding air. Meanwhile the barometer and attached 

 thermometer were read. Then the Regnault hygrometer was 

 set up on the bench, which was adjusted to suit the wind ; the 

 wet-bulbs were thoroughly wetted by raising the water-vessel, 

 which was then removed. The aspirator was started, and the 

 wet-bulbs watched ; after some fluctuation a minimum steady 

 temperature was attained, which was noted. The dry bulb 

 was immediately read, and the dew-point determined as rapidly 

 as possible ; since the temperature of the ether had been 

 lowered while the wet-bulbs were being watched, the whole set 

 of observations could usually be obtained in well under a 

 minute. (The dry bulb is found to attain a steady tempera- 

 ture more quickly if it is dipped in the water for a few 

 moments and then carefully dried before placing in the 

 thimble ; the current of air draAvn past it ensures that it is 

 always in contact with the outside air. At the beginning of 

 the series an extra thermometer was hung outside with the wet- 

 bulbs, and showed no divergence from that in the hygrometer). 

 The direction and force of the wind were also noted — i.e., 

 whether light, strong, etc. As far as possible, observations 

 were taken twice a day, at about 11 a.m. and 3 p.m. Those 

 here tabulated were taken between May 8 and July 4 inclusive. 



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