REPORT OF COMMITTEE ON OBSERVATORIES 85 



showers and the disturbed atmospheric conditions indicated would 

 without doubt prove an obstacle to the prosecution of solar work in 

 this region. 



The thermographic record obtained by the expedition at Cuya- 

 maca extends over twelve days, from May 27 to June 8, inclusive. 

 The temperature curve for this period is generally quite smooth and 

 free from rapid alternations, but has, as a rule, a large daily range. 

 This sometimes amounts to as much as 35 °, passing in one instance 

 from 46 at 6 a. m. to 81 ° at 4 p. m., and in another from 43 at 5 

 a. m. to 78 at 5 p. m. It was particularly noted that the only times 

 of constant temperature were those when a strong sea breeze was 

 blowing. Nevertheless, at such times, the seeing was invariably 

 poor. The temperature curve at night was generally steep, often 

 having a declining rate of from 3 to 4 degrees an hour. Only once 

 was there a period of constant temperature at night. This lasted 

 about four hours, with a variation of about i°. 



A thermograph was placed on North Cuyamaca peak on May 29, 

 and allowed to remain there in a suitable shelter until June 8, thus 

 giving a record for ten days. This record shows that more fre- 

 quent small alternations of temperature were taking place on the 

 summit than at the lower station, producing a less regular curve, 

 especially during the day. The daily range in temperature on the 

 summit was, however, much smaller. As a rule, it was less than 

 20 ; the maximum was 29 . The night temperatures on the 

 summit were quite satisfactory. On most of the nights there were 

 intervals of several hours during which the temperature remained 

 nearly constant or fell very slowly. 



During the time I was at Cuyamaca the dews were heavy. This 

 is a normal summer condition, due to the rapid fall of temperature 

 at night. 



The observations at Echo mountain and Inspiration point on 

 Mount Lowe had shown that excellent night seeing prevails there. 

 The day seeing there was also good at times, but it did not have that 

 constant excellence which was deemed desirable. 



Mount Wilson is only 3 miles east of Mount Lowe, and there was 

 every reason to think that it has the same excellent night conditions 

 that prevail on Mount Lowe. On the other hand, it appeared prob- 

 able that it would have better day conditions, owing to the control 

 of insolation and radiation by its dense covering of evergreen trees 

 and chaparral. On going to Mount Wilson, therefore, I was in- 

 structed to devote the greater portion of the time to testing the see- 



