MOUNT WILSON SOLAR OBSERVATORY. 



263 



INVESTIGATIONS OF STARS AND NEBUL/E. 

 OBSERVING CONDITIONS. 



Observing conditions at night for the year ending August 31, 1915, 

 were somewhat less favorable than during the two pre\'ious years. 

 The 60-inch reflector was in use 153 entire nights and during a part of 

 111 nights, while on 101 nights no observations could be made. Out of 

 3,591 hours of darkness the instrument was in use 2,006 hours, or 55.9 

 per cent of the total night-time. There were 1,585 hours when obser- 

 vations v/ere interrupted because of weather conditions and 56 hours for 

 silvering, repairs, etc. Table 1 gives the statistics for each month: 



Table 1. 



The total exposure time for the year was 69 per cent of the observing 

 time and 40 per cent of the night-time. Average exposure per plate, 42 

 minutes. 



As usual, the month of August gave the highest percentage of good 

 weather, only 3 hours being lost on account of clouds. This month 

 also gave very little wind, which was light on 9 nights and moderate 



Table 2. 



Wind percentage: High, 9; brisk and moderate 26; light to calm, 65. 



