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Proceedings of the Royal Soeiety 
of the air we may have at the same temperature very different read- 
ings of the wet bulb, according to the rate at which the air may 
happen at the time to circulate. The quicker the circulation, up 
to a certain rate, the lower will he the temperature indicated. 
Now it is evident that if the readings are correct when taken 
in calm air, they must be incorrect when there is a rapid circula- 
tion; and as the rate at which the air circulates in screens is 
constantly varying, the observations made with the arrangements at 
present in use are of little value. 
The reason for the difference in the readings taken under the dif- 
ferent conditions seems to he that, when there is no circulation, the 
atmosphere round the wet bulb is more moist than the surrounding 
air, owing to the evaporation, and the wet bulb thus indicates a high 
humidity ; but when the moist air is removed as quickly as it is 
formed, the temperature falls lower. In illustration of the effect of 
the greater or less freeness of the circulation on the readings of the wet 
bulb, and to show the small value of the indication of some of the 
instruments at present in use, I will give the readings of a combina- 
tion of wet and dry bulb thermometers which is now before 
me. The instrument is one of a kind which is frequently used. 
The two thermometers are fixed to a piece of boxwood, on which 
the scales are marked. The bulbs of the thermometers project 
some distance below the edge of the scale. The thermometer and 
scale are carried in a metal frame which supports the water bottle 
for moistening the wet bulb. The observations were made in a 
room, to get constant conditions of temperature and moisture. The 
instrument was hung clear of everything, to allow of a free circula- 
tion at the back. Under these conditions the readings of the 
thermometers were — dry bulb 62°, wet bulb a little under 59°. 
The thermometers were now taken out of the frame, and hung up 
in the same place ; the increased freedom of circulation obtained 
by removing the frame made the wet bulb fall to 57°. The 
instrument was now placed in a strong draught of air, produced by 
a revolving fan, when the wet bulb fell to 56°, the dry one re- 
maining all the time at 62°. Using this instrument as was in- 
tended by the maker, it showed a humidity of about 80. When 
stripped of its frame and the air allowed to get freely at the wet 
surface, it showed a humidity of 72, and when put in a current 
