208 Love and Smeal : 



the effect of different influences on the psychrometer, artificial 

 regulation may be best, but the applicability of the results in 

 practice must be tested under the conditions in which meteoro- 

 logical observations are usually taken. The ordinary types 

 of thermometer-screen were not, however, suitable for the 

 purpose, since it was desired not only to use several wet-bulb 

 thermometers, but also to directly determine the humidity by 

 another method. For these reasons a new screen was designed 

 by one of us (E.F.J.L.), which is depicted in Plate XLYIIL, 

 looking respectively N.W. and S.W. towards the screen. It is 

 large enough to allow the observer to be inside and yet well 

 away from the instruments, and by means of a sliding bench 

 it is always possible to keep to leeward of them, so that the 

 effects of radiation from the person are as small as jDOssible. 

 Full protection from the sun's direct rays is afforded, and 

 a double roof prevents heating from that direction, the air 

 circulating freely between the roofs. The force of the wind is 

 broken by louvring as shown, the ventilation being always 

 ample. The dimensions of the screen internally are 65 in. (165 

 cms.) each way, the height running from 65 to 87^ in. (165 to 

 222 cms.). A trolley traverses the whole length, carrying n 

 bench 48 in. (122 cms.) long, at a height of 50 in. (127 cms.) 

 from the ground. The instruments are placed on this bench, 

 which is adjusted to be out of the sun's rays and to windward 

 of the observer. The outer roof is 12 in. (30.5 cms.) above the 

 other. 



By the kind permission of the University CouuLil, and the 

 generous provision by Professor Lyle of the necessary funds 

 from the apparatus grant of the Natural Philosophy Depart- 

 ment, the screen was set up in a suitable position in the 

 grounds, the outlook being open on all sides, except for a wall 

 distant 11 yards (10 metres) on the east. The prevailing winds 

 are north and south, hence the screen was set facing south, the 

 roof sloping downwards to the north. 



Preliminary work was carried on l)y Mr. T. C Sutton, 

 B.Sc, in 1909, with degree thermometers. Eleven thermo- 

 meters of precisely similar size and shape were carefully com- 

 pared and calibrated, of which seven were employed as wet- 

 bulbs, and were covered respectively with silk, linen. wo(j1 



