[btarkey & BARNES] DEFICIENT HUMIDITY OF THE ATMOSPHERE 191 



In the first column of the table we give the type of instrument. No. 

 1 wet and dry represents our original hygrometer with the very small 

 bulb which gives high values. No. 2 wet and dry is the second hygro- 

 meter with the larger bulb, which we found much more satisfactory. 

 No. 3 wet and dry Ave do not include here because it never differed to 

 any extent from No. 2. This shows that there must be some limit to the 

 bulb surface for accurate readings. Thus a small surface does not 

 permit of sufficient evaporation to cool the glass and stem enough to 

 offset the attainment of heat from the warm room of the laboratory. 

 The wick and covering of this hygrometer were altered several times, 

 but in no case did its indications approach the true ones. This shows, 

 we think, the importance of checking a wet and dry bulb instrument 

 before it is used by an absolute instrument like the Eegnault or the 

 chemical hygrometer. 



The second column of the table gives the date, and the third gives 

 the maximum and minimum air temperature, which of course determines 

 the dryness indoors. The fourth column' gives the air temperature of 

 the laboratory at the time of experiment. 



The fifth column gives the relative humidity, calculated from 

 Glaisher's tables, in the case of the wet and dry bulb instruments ; and 

 from the vapour pressure at the dew-point for the Eegnault hygrometer. 



The dew-point, as found on the silver surface, is given in the sixth 

 column. 



An interesting question arises here as to the deposition of moisture in 

 the form of vapour or hoar-frost. The vapour pressure of hoar- 

 frost is considerably smaller than that for supercooled water. The use 

 of the vapour pressure of water in place of frost for calculation would 

 give results as much as 25 per cent too high at 0° F. 



In the last column of table No. 2, we remark on the observations for 

 the Eegnault, where we thought hoar-frost was deposited, or where we 

 had supercooled water. 



We think there is no question that for very low observations of the 

 dew-point, we got in all cases hoar-frost. In some of the higher ones 

 there was a doubt, and we believe that this will introduce some error 

 for temperatures not far below 32° F. 



Our chief difficulty always lies in reconciling the results of the 

 chemical estimation Avith those of the methods involving the deAV-point, 

 throughout the whole range of temperature and degrees of humidity. 



As has been seen from table No. 2, conditions have been arrived at 

 where the different instruments, all involving a dew-point, can be made 

 to agree approximately as to their readings, but when Ave turn to the 

 chemical results, there we get marked divergence in some cases. 



