MOUNTAIN OBSERVATORIES IN EUROPE AND AMERICA. 53 



of a change of season. This day may be followed by weeks of good 

 weather, with only an occasional foggy afternoon and night. 



Such is a very succinct account of the average summer day. There 

 ai'c exceptional days of fog at the summit, of no fog in the valley, and 

 days when the trade-winds (which bring fog) are replaced by northers ; 

 but, in general, the above account represents the typical summer 

 day. During all the summer the vision is, as a rule, steady, and for 

 very many days it is remarkably so. If 3 represents the average 

 seeing of the year (on a scale of 1 = very poor, to 5 = perfect steadi- 

 ness) the days of Jane and early July are apt, in the long run, to be of 

 gi'ade 4, while late July, August, and part of September are of even 

 higher excellence, and contain a considerable number of nights of per- 

 fect, or nearly perfect, seeing. In this brief account allowance must 

 be made for exceptions, but, in the large, the course is about as just 

 recited. If the seeing is good for part of a night it is usually good 

 throughout. 



In seeking for a cause of the excellent conditions of vision which 

 })revail liere during the summer nights, it appeared to me to be 

 largely due to the banks of fog which lie over the Santa Clara valley. 

 During the day that valley and our surrounding foot-hills, etc., are 

 intensely heated. The thick layer of fog which often covers them from 

 sunset onwards acts as a screen to shut in the radiations, and to allow 

 the higher summits (as Mt. Hamilton) to quickly cool, and to take the 

 temperature of the superincumbent air. 



There is no doubt whatever that our best nights usually follow days 

 when the fog has covered the valleys (though there are exceptions). 

 The very best vision is usually accompanied by calms or very light 

 airs. There is no doubt whatever that when we can see the electric 

 lights of San Jose some thirteen miles away and 4000 feet below us 

 (and thus when there is no fog) the nights are usually not of first-class 

 excellence (though there are exceptions).* 



During the years 1881-88 my stays at Mt. Hamilton vcere seldom so 

 long as a month. The exceptional cases did not then impress me, and 

 I was disposed to attribute the excellence of our summer vision 

 almost entirely to the presence of the low-lying sea fogs. I still think 

 that they are the principal factor ; but an experience of actual residence 

 during the years 1888-1896 has proved to us all that the explanation 



* When the lights of San Jose can be seen they are usually very unsteady, as 

 might be expected, and the vision at Mount Hamilton is usually not good, as I 

 have said. There are, however, occasions when the San Jose lights appear quite 

 steady and yet are clearly seen ; and on such occasions the vision at Mount 

 Hamilton is usually good, though beidora perfect. The latter conditions are 

 usually accompanied by a calm. 



