46 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Comparisons between dry bulb thermometers, Green, 7d9,p}aced in the instrument shelter, and Green, 

 811, read afttr being swung rapidly through the air. 



[These comparisons were made on dift'ercnt days betweeu April 25 aud May 2. The swinging of Grekx, 811, was 

 repeated seTiral limes until it seemed certain that it had assumed its true reading. The dirterencesonly between 

 the readings are given. The instrumental corrections to both of these thermometers are O.O.] 



811—799. 



If we assume, as is usual, that the whirled thermometer gives a close approximation to the 

 true air temperature, the above comparison shows what correction should be applied to the ther. 

 mometer in the instrument shelter. It will be seen that the effect of placing a double roof upon 

 the shelter was to diminish this correction, the correction becoming almost inappreciable except in 

 the extreme case on May 2, when there was no wind and the air was accordingly stagnant in the 

 shelter. As a result of these comparisons, it was decided to adopt the readings of the dry -bulb 

 thermometer within the shelter without any correction for the air temperature. 



APRIL 25, 1883. 



Miscellaneous. — Minimum, 78.2. Minimum at ground, 76.6. Maxinmm, 88.2. 

 6.00 a. m. Double rainbovr in the west. 



8.30 p. m. Zodiacal light plainly seen in the west, but with limits ill defined. It extended about 30° in altitude, 

 60° in azimuth. 



9.10 p. m. Moon made shadow bands through the clouds above it, similar to those frequently made by the sun. 



