MOUNT WILSON SOLAR OBSERVATORY. 



213 



characterized by an exceptional amount of cloudiness. The 60-inch 

 reflector was in use 188 entire nights and parts of 81 nights. No 

 observations were made on 96 nights. Out of 3,591 hours of darkness 

 the instrument was in use 2,272 hours, or 63 per cent of the time. The 

 statistics for each month are given on the preceding page. The mir- 

 ror was resilvered in November and April. 



The total exposure time for the year was 75 per cent of the observing 

 time and 47 per cent of the hours of darkness. The observing con- 

 ditions as regards seeing (on a scale of 10) and wind velocity are given 

 in the accompanying table: 



A standard self-recording anemometer provided by the United 

 States Weather Bureau was installed on Mount Wilson in April and 

 has been in service since that time, furnishing continuous records of 

 the wind velocity. Since May 1 the highest recorded velocity during 

 any five minutes was 30 miles per hour on May 5. The average 

 velocity May 1 to August 31 was 6.7 miles per hour. 



The precipitation during the year was 28.41 inches, the largest 

 amount occurring in January. The snowfall amounted to 72 inches. 

 Much fog occurred during the spring months, especially in the month 

 of May. The maximum temperature was 98° F. on June 17; the 

 minimum 10° F. on January 16. 



PHOTOGRAPHS OF NEBUL.^ AND CLUSTERS. 



Photographs of the following spiral nebulse were made during the 

 year by Mr. Ritchey at the primary focus of the 60-inch reflector; 

 the exposure times varied from 80 minutes to 465 minutes: N. G. C. 

 628, 650, 891, 3031, 5194-5, 6412, 6946, 7331, 7814. A close group 

 of five spiral nebulae in Pegasus was also photographed. 



An interesting feature of the results w^as the discovery by Mr. 

 Ritchey of a star on the photograph of the spiral nebula N. G. C. 6946 

 which did not appear on five photographs taken between 1910 and 

 1916. Several additional negatives confirmed the reality of this nova. 

 The star was of about the fourteenth magnitude on July 19, 1917, 

 and about one magnitude fainter on July 27. It occurs in one of the 

 outer arms of the spiral at a distance of 2' from the nucleus. A spec- 



