WEATHER BUEEAU. 265 



tween the prevailing weather conditions and the amount and quality 

 of change of temperature to be expected in the valleys below. Indi- 

 rectly, the data secured by these studies have an important bearing 

 on the question of air drainage, and consequently on the protection 

 of fruit in tlie valleys below. 



Solar radiation investigations. — The gradual extension of this 

 work is continually demanding new and improved instruments, con- 

 sequently considerable time is devoted to developing and testing 

 apparatus. The Mount Weather Observatory now has the follow- 

 ing equipment for the measurement of the intensity of solar radiation 

 and the polarization of sky light : 



One Callendar pyrheliometer, exposed horizontally above the roof 

 of the physical laboratory. 



Two Callendar pyrheliometers, one with the glass cover removed, 

 and both mounted equatorially and clock driven, and shielded from 

 diffuse sky radiation by means of diaphragmed tubes. 



One Leeds & Northrup recording Wheatstone bridge. 



Three Marvin pyrheliometers having different types of thermal 

 elements, with auxiliary apparatus. 



One Smithsonian silver disk pyrheliometer. 



One Angstrom pyrheliometer, reserved for comparative readings. 



One Angstrom pyrheliometer with absorption screen. 



Two Pickering polarimeters. 



Two Savart polariscopes. 



Measurements of the polarization of sky light, of the positions of 

 the neutral points of Arago and Babinet with the sun near the hori- 

 zon, and of the intensity of solar radiation, have been made at Mount 

 Weather throughout the year whenever atmospheric conditions per- 

 mitted, except when the observer was absent from station on other 

 duties. Polarization measurements were obtained on 63 days and 

 radiation measurements on 73 days. These measurements are em- 

 ployed in studies of atmospheric transmissibility, which not only 

 varies with the weather conditions from day to day, but also varies 

 from year to year, on account of causes not yet well understood. 



Since May 16, 1912, a continuous record has been maintained by 

 means of the horizontally exposed Callendar pyrheliometer and one 

 of the recording bridges, of the intensity of the radiation received 

 from the sun and sky upon a horizontal surface. By means of the 

 other recording bridge and one of the equatorially mounted Cal- 

 lendar pyrheliometers occasional records have boon obtained of the 

 intensity of direct solar radiation. At the same time the intensity of 

 direct solar radiation has been measured either by a Marvin pyrheli- 

 ometer or the Smithsonian silver disk pyrheliometer, or both. Com- 

 parisons of the records of the two automatic instruments give 

 measurements of the intensity of the radiation received ditlusely 

 from the sky upon a horizontal surface. Comparisons of the records 

 from the equatorially mounted Callendar pyrheliometer and the 

 measurements by the Smithsonian silver disk pyrheliometer indicate 

 a difference between the Smithsonian and the Cnllendar standards. 

 However, the comparisons, at least, make it possible to express all the 

 measurements in units of the same value. 



Continuous records were obtained by means of a horizontally ox- 

 posed Callendar pyrheliometer throughout the year at Madison, Wis., 



