104 CARNEGIE INSTITUTION 



I exposed it in the thermograph shelter of the L,owell Observatory. 

 The range of temperature shown by the record was 29 , from 59 

 to 88°. The greatest variation in any one day was 24 . 



The record during the day was generally full of small variations, 

 in addition to the larger changes. This was probably due to the 

 passing of more or less extended clouds, giving intervals of sunshine 

 and of shade, but no direct test of this hypothesis was made. 



The temperature at night was more uniform than by day, but the 

 night curve did not always have the smoothness that one likes to 

 see. However, on some of the nights there would be an interval of 

 five or six hours during which the change of temperature would 

 amount to only one or two degrees. 



Mr. Lowell very kindly placed at my disposal the thermographic 

 record of the observatory. The time available did not permit a de- 

 tailed study of the entire record, but I examined with some care the 

 sheets for the year 1902. 



Some of the general characteristics of the record were at once 

 evident. The fluctuations during the daytime were often large, and 

 superposed upon these were frequent minor variations. In passing 

 from day to night temperatures there was, as is always the case, a 

 pretty rapid change during the early hours of the night. This 

 period usually lasted about three hours — to take the year through, 

 from about 6 p. m. to 9 p. m., but earlier in winter and later in 

 summer. This, however, was not the rule. After the early even- 

 ing hours the temperature gradient would generally become less 

 steep, or even nearly horizontal, save for minor fluctuations. 



A feature worthy of special notice was prominent on many of the 

 sheets. Often in the latter part of the night, generally after 3 a. m. 

 and often as late as 6 a. m., the temperature would suddenly fall 

 from 5 to 15 , and then after a short time rise again, partially re- 

 gaining its former height. As an example, on the sheet I obtained 

 there is shown on the morning of July 14 a sudden drop of about 

 7 shortly after 3 o'clock a. m , and half an hour later a rise, nearly 

 as sudden, of about 4 . I have been told that these sudden changes 

 of temperature are here associated invariably with very bad seeing, 

 which is entirely in accord with what might be expected under such 

 circumstances. 



Respectfully submitted. 



W. J. Hussey. 



Mount Hamilton, Cal., 

 August, 1903. 



