Psyclirometric Forimda: 209 



(worsted), flannel, muslin, cotton-wool, and a mixed tussore 

 fabric ; the materials were first cleansed from grease, starch 

 and other impurities by boiling- in soda and distilled water. 

 The whole eleven thermometers were suspended in a row above 

 the bench, with the four dry-bulbs in the positions 1, 4, 8 and 

 11, the row being at right angles to the wind. Observations 

 were only taken when the four dry-bulbs showed the same 

 steady temperature, since then the intermediate wet-bulbs could 

 be assumed to be under identical conditions. While the read- 

 ings of the wet-bulbs were being taken, an observation of the 

 dew-point was taken with a Regnault hygrometer. In all some 

 sixty (60) observations were made, and Mr. Sutton reports that 

 a striking feature is the consistency with which the various 

 wet-bulbs rise and fall together with fluctuations in the 

 humidity, etc. He also states : *' In no case was there the 

 slightest sign of any difference of temperature between the 

 bulbs, so long as the water used was pure, and the terminal 

 dry-bulb thermometers indicated the same temperature." These 

 results showed that the difference, if any, was decidedly less 

 than 0.1° C for dry temperatures ranging between 17° and 

 41.7° C, Tlie next year, Mr. Sutton, being in England, made 

 some more observations in a sheltered spot in the open at 

 temperatures between —4° and 10°, at Twickenham, Middle- 

 sex. The differences in this case were certainly less than 0.2°, 

 the limit of accuracy of the thermometers employed. 



We were thus encouraged to proceed with the Avork in ear- 

 nest this year. The values of the constant A obtained- by Mr. 

 Sutton showed a marked dependence on the velocity of the 

 wind, suo-o'esting some >light alterations in the construction of 

 the screen which were carried out before beginning the new 

 series of observations. 



Apparatus and Procedure. 



Three wet-bulb thermometers were used, all of Jena glass, 

 graduated in tenths of a degree Centigrade, and with cylindrical 

 bulbs. The materials were (1) silk, (2) linen, (3) cotton. Un- 

 fortunately the silk-covered one was broken after the work had 

 been proceeding for some time, and before calibration had been 



