544 



MR A. J. HERBERTSON ON 



Dry and Wet Bulb Thermometers, and Mass of Water Vapour. 



Instead of determining the relationship between dry and wet bulb readings and 

 those of a condensation-hygrometer in moving air, and thence deriving the vapour 

 pressure, comparisons may be made directly between the dry and wet bulb temperatures 

 and the mass of water vapour arrested by some desiccator through which the air is 

 drawn ; and this was carried out in the present research. 



The ordinary meteorological elements were observed in the ordinary way during each 

 experiment, and an Assmann aspiration-psychrometer was used to obtain dry and wet 

 bulb readings, in addition to the stationary thermometer in a Stevenson screen. As a 

 rule, temperature readings were taken every minute during each experiment. 



A 6-inch U-tube, with a small bulb blown at the bend and an L piece of narrower 

 tubing fused to one arm, was filled with granulated pumice, which had been washed 

 with sulphuric acid and then heated until fumes ceased to be given off. A second 

 piece of narrow tubing, shaped C, was next fused to the free arm (see fig. 2, a). 

 Afterwards pure sulphuric acid was sucked in, and allowed to drain off; and the narrow 



W 



To Aspirator. 



Fisr. 3. 



part of the tube was cleaned. After trying various expedients, the tubes were stopped 

 by a glass cover held in its place by a narrow band of india-rubber, which prevented 

 any leakage (see fig. 2, b). The tubes were joined by keeping the free ends close 

 together by means of a piece of tightly fitting india-rubber tubing (see fig. 2, c). 



Three capped tubes were weighed, one against weights, and each of the other two 

 against this standard tube and weights combined. The standard tube went through 

 the same experiences as the other tubes, except that the caps were never removed. 

 The other two tubes were then joined in series, and a third tube attached which pre- 

 vented any vapour from the aspirator reaching the weighed tubes. Whenever the 

 second of the two tubes began to gain more than a few tenths of a milligramme in 

 weight, the first tube was rejected and a new one inserted. It was found advisable to 

 draw air through a new tube, or one which had not been in use for some time, 

 before using it in any determination. 



In the earliest experiments, Muencke's reversible aspirator or a 14-litre bottle were 

 used as aspirators.* 

 * Cf. Proceedings R.S.E., xx., p. \11 ff., where tables and account of preliminary experiments will also be found. 



