<jmd the Psychrometer. 443 



and below the bulb. The water used should be rainwater, if 

 possible. In the winter season, or when the wet bulb tempera- 

 ture is below freezing, great care must be taken in coating the 

 bulb with a uniform layer of ice. To do this properly, it is 

 necessary, whenever it is found that the air temperature is 

 going below freezing, to take the wick out of the cup and 

 hang the free end over the support behind ; it should not be 

 taken off the thermometer. If this wick becomes stiffened 

 with ice before it has been taken out of the cup and cannot be 

 bent, the whole, including bulb, cup and wick, must be im- 

 mersed in luke-warm water, lifted up from beneath, till the 

 wick is sufficiently pliable to permit properly adjusting as 

 above. To wet the bulb, keep the cup of water as near 32° as 

 possible and 10 to 25 minutes before the observation, if there 

 is no artificial ventilation, lift the cup up from beneath and 

 wholty immerse the bulb; in withdrawing the cup, wait a 

 moment after the bulb is exposed and touch the drop that will 

 be hanging from it, with the edge of the cup. If there is a 

 high wind, two or three coats at once may be needed to obtain 

 a good result. A mass of ice should not be allowed to accum- 

 ulate on the stem or about the bulb, but it should be melted 

 off and a uniform coat applied. No brass scale or protector 

 should ever be allowed within 1*5 inches of the wet bulb. 

 This scale has the temperature of the air, and if the muslin or 

 wick touch the scale its temperature will be conducted to the 

 bulb, thus giving too high a reading. If the muslin becomes 

 brown or covered with dust it should be replaced by clean; 

 this may need to be done once a month in some places. 

 Frequently the muslin will be found rather firmly fastened to 

 the bulb by the formation of a lime-like substance ; in such 

 case, the bulb may be cleaned by using a bath of dilute muri- 

 atic acid. 



When the temperature reaches about 20° there seems to be 

 a peculiar effect produced which causes the indicated tempera- 

 ture to be too high and which increases as the temperature 

 falls. This may be due to a contraction of the ice-film or coat- 

 ing ; at 0° the effect amounts to nearly 5°, but after that point 

 is reached the ice seems to partly give way, as the contraction 

 seldom goes beyond 5°. Experiments have been made by 

 immersing dry and ice-covered bulb thermometers in a bath 

 of mercury and also in disulphide of carbon with identical 

 results. As long as the temperature was lowered or remained 

 stationary, even down to —20°, there was still a marked effect 

 observed, but the moment the temperature began rising, possi- 

 bly because the ice expanded so much more rapidly than the 

 glass, the two thermometers began to approach each other and 

 quickly agreed." The above explanation seems extraordinary 



