[STARKEY A BARNEs] DEFICIENT HUMIDITY OF THE ATMOSPHERE 189 



These observations were taken on a day when the Eegnault instru- 

 ment gave a relative humidity in the laboratory of somethmg over 6^. 



No. 1 shows a smaller difference between the wet and dry bulbs than 

 No. 2, and appetirs to give always a higher value of the humidity. 

 This we attribute to the small bulb it possesses, compared to tlie size 

 of the theromometer and wick. 



No. 2 shows a large enough difference to give a correct value of the 

 hiimidity=63/2 per cent. 



No. 3 instrument, with the cylindrical bulb, aJso gave correct indica- 

 tion in the draught, aoid agreed with the No. 2, 



A wet and dry bulb sling hygrometer was specially made for us in 

 Germany, and was included in these comparisons. 



This type of instrument is now generally used, except where con- 

 tinuous observations are required, and for that reason a record of its 

 readings under different conditions is interesting. In the first place 

 thei wick was coiled round the lower part of the stem of the thermometer 

 just above the bulb, so that when slung the water descended from the 

 wick into the covering over the bulb. 



When exposed to the rapid rotation it was found that no steady read- 

 ing of the difference could be obtained before the water had become 

 so completely evaporated as to be no longer useful. The difference 

 gradually became greater after each series of rotation until it indicated 

 a relative humidity much lower than that calculated by the dew point 

 hygrometer. 



This defect was rectified by protecting the moist wick above the bulb 

 from evaporation, by a covering of tin-foil. After this was done repeated 

 rotation did not alter the relative differences, and th.e. readings agreed 

 closely with the Regnault hygrometer. This fact is an important one 

 to bear in mind when using this instrument, and shows that the correct 

 difference between the wet and dry depends upon keeping the feed- 

 water as nearly as possible at the air temperature, or that of the dry 

 bulb. 



This may be shown in the case of a stationary instrument by heating 

 or" cooling the feed-water. 



We have found as a result of our tests on a suitable wet and dry bulb 

 instrument, both stationary and sling, that correct indications can be 

 obtained for very low relative humidity in a warm room, when Glaisher's 

 tables are used. As we have no record of Glaisher's procedure when 

 compiling his tables, we presume that the differences in the columns 

 between the wet and dry bulbs are those for the feed-water at the tem- 

 perature of the dry bulb. 



Feed- water at a temperature lower tnan this will, in a dr}-- atmos- 

 phere, cause a considerably larger difference. This discrepanc}^ is met 



