and the Psychrometer. 445 



it a thermometer for obtaining the air temperature at the same 

 time, the latter being an important consideration. Most careful 

 experiments have been made looking to the determination of 

 all the errors to which this sling psychrometer may be supposed 

 to be subject. At the greatest velocities attainable by hand, 

 no harmful effects have ever been noted from friction with the 

 air or from expansion of the bulb through the centrifugal action 

 of the mercury column. Any convenient velocity may be 

 used; about four meters per second, ten miles per hour, was 

 found satisfactory. The results were precisely similar at all 

 velocities from two to ten meters per second, great care being 

 taken in wetting the bulb uniformly during the entire time of 

 swinging, and also continually changing the plane in which the 

 bulb was swung. Eepeated trials in' a room have shown that 

 an accuracy of a tenth of a degree may readily be attained with 

 this device in one minute. In the open air it is necessary to 

 swing out of sunshine and in a moderate shade only, if the air 

 temperature is desired. It has been thought by some that at 

 night with a clear sky, radiation from the bulbs into space is 

 harmful, but experiment shows that the amount of this radia- 

 tion from bulbs suspended in a still air is very slight, and the 

 rapid motion entirely annuls any evil effect. For the best re- 

 sults it is essential that the sling wet bulb should be continu- 

 ously moistened. To do this properly requires some skill and 

 constant watchfulness. The muslin should be carried up the 

 thermometer stem about an inch, and to the latter there should 

 be fastened a tube of glass drawn out to a point. The point 

 should be placed just at the top of the muslin, or at least one 

 inch from the bulb, otherwise its heat imparted to the water 

 will tend to raise the temperature. A string inserted in this 

 tube may be pressed down or pulled out according as the air is 

 moist or dry, the intention being to keep the muslin constantly 

 and uniformly moistened. 



Comparisons. 



The investigations of others have established a formula for 

 comparing these instruments, which may be taken as a work- 

 ing hypothesis, as follows, for temperatures above freezing : 



x=f— 4 (t— t') Ah, in which 



x= the tension of vapor of saturated air at the dew-point temp. 

 f— the tension of vapor of saturated air at the wet-bulb temp. 

 t= air temperature. 

 h=- air pressure in inches of mercury. 

 A= a constant to be determined. 



For temperatures below 32°, it has been thought by some 

 even to the present time, from theoretical considerations, that 



