198 CARNEGIE INSTITUTION OF WASHINGTON. 



The apparatus is fairly sensitive, giving a deflection of about 25 

 divisions when one cell is exposed to the image of the moon at last 

 quarter, the other to the neighboring sky (in daylight) . At maximum 

 sensitiveness the insertion of a piece of clear plate-glass above one 

 cell, producing a loss of 8 or 9 per cent, causes a deflection of about 

 80 divisions of the scale. At this sensitiveness, however, there are 

 continual fluctuations of 5 or 10 divisions near the sun, making the 

 measurement of very small differences impossible. The work will 

 be continued with improved apparatus. 



It is perhaps barely possible to send meteorological balloons, carry- 

 ing automatic photographic apparatus, directed toward the sun by a 

 gyrostat, and finally controlled automatically by the heat of the sun's 

 image, to a height sufficiently great to allow the corona to be photo- 

 graphed without an eclipse. But the technical difficulties would be 

 considerable. 



INVESTIGATION OF STARS AND NEBULAE. 

 OBSERVING CONDITIONS. 



The weather conditions for observations at night on Mount Wilson 

 during the year ending August 31, 1918, were above the average. 

 The months September to December 1917 inclusive gave an unusual 

 amount of clear weather. Observations were carried on with the 

 60-inch reflector during all of 206 nights and a part of 91 nights, while 

 on 68 nights no observations were made. Out of 3,591 hours of 

 darkness the telescope was in use 2,418 hours or 67 per cent of the 

 time. The total exposure time for the year was 71 per cent of the 

 observing time and 48 per cent of the hours of darkness; 63 per cent 

 is the average observing time during the hours of darkness for the 

 past six years. 



The mirror was resilvered in September, 

 April, and August. Statistics for each 

 month, the conditions of seeing (on a scale 

 of 10), and the wind velocity as noted each 

 night are shown in the accompanying tables. 



The self-recording anemometer instaUed 

 on the Snow telescope building last year was 

 moved on July 10, 1918, to the top of the 

 150-foot tower. This location, which is 

 nearly 100 feet higher than the former one, and well above all ob- 

 structions, gives a more accurate record of the air-movement over 

 the mountain-top. Readings made so far indicate an increase of from 

 40 to 50 per cent in the mean annual velocity, which at the old loca- 

 tion was about 7 miles per hour. The highest velocity recorded during 

 the past year was 36 miles on March 7 and April 19. The total 

 precipitation for the year was 28.93 inches and the total snowfall 

 34.5 inches. The highest temperature was 97° F. on August 3, the 

 lowest 18° F. on February 19. 



