144 



PROCEEDINGS OP THE AMERICAN ACADEMY 



February 28, 1888. 

 Barometer 30.35. Thermometer — 18° C. 



To illustrate the effect of the weather upon such observations as 

 these, we may compare with the above the following table, made up 

 from the results of October 21, 1888. On the former occasion the air 

 was very calm, and from its low temperature ( — 13° C), must have 

 been quite dry. The evening of the latter date was fine, after many 

 days of fog and rain, and, although clear to the eye save for the merest 

 visible milkiness, the drifting clouds of vapor were made manifest by 

 the irregular transmission of the lunar rays. 



P'ollowing Langley,* we may find the approximate transmission 

 coefficient for a column of air capable of supporting one decimeter of 

 mercury, by employing the formula 



t = (M,8^_ - M,/30 |/ 



* American Journal of Science, vol. cxxv. p. 176. 



