52 IMOUNTAIN OBSERVATORIES IN AMERICA AND EUROPE. 



can now be spared for such experiments. Even if tliey are only par- 

 tially successful the conditions of dtiyliglit vision will be improved. 



Observations which can be made at almost any hour of the day (as 

 spectroscopic and photographic observations of the sun) can usually, 

 by diligence, be well made here, by choosing the best moments. The 

 long series of solar photographs (many being excellent) obtained here 

 by Mr. Perrine prove that such moments can be selected. 



The negatives of the Transit of Venus of 1882 which were secured 

 at Mt. Hamilton (by Professor Todd) are said to be the best of any 

 which were measured at Washington. 



Observations which must be made at a fixed instant (as meridian- 

 transits of sun and stars) are, on the whole, not likely to be observed 

 under as good conditions here as at the average observatory in the east. 

 As the slopes of the plateau become covered with vegetation these diffi- 

 culties will be in part removed, but in part only. 



The effect of fog in the valleys on good vision : During rny 

 visits to Mount Hamilton in the years 1881 and subsequently I 

 made careful notes of the conditions of good vision at the sum- 

 mit. The phenomena of an average summer day occur somewhat 

 as follows : The sun rises in a 'clear sky, and no clouds are visible 

 dilring the entire twenty-four hours. The days are hot (though, of 

 course, not oppressive, since the air is exceedingly dry), and the vision 

 during the day is usually unsteady on account of air-currents which 

 rise from the neighboring canons and from the heated rocks immedi- 

 ately bordering the plateau of the summit. The large valley of Santa 

 Clara lies all day long in the ardent sunshine. Late in the afternoon 

 the sea fog begins to creep in through the various gaps in the coast- 

 range (which borders the Santa Clara valley on the west) and to come 

 towards the north from Monterey Bay through the valley of the Pajaro 

 River, etc. A local fog from the Bay of San Francisco often sends ite 

 thinner veils towards the south, but seldom extends as far south as 

 Gilroy, where the Monterey fog is entering. Up to this time the fogs 

 are low-lying. As the afternoon goes on the sea-fog rises higher and 

 higher, and often pours over the tops of the highest peaks of the Coast- 

 Range (3000 to 3800 feet), and completely covers the valley of Santa 

 Clara, and fills our neighboring canons close below us, but seldom (in 

 summer) rises to our own level. Usually it is say 1000 feet lower. 

 The hotel at the foot of the mountain ha.^ frequent fogs in summer (it is 

 2000 feet lower) when Mt Hamilton has none. 



The night fogs (in summer) seldom rise to the summit, and they 

 are dissipated in the early morning. 



As the autumn comes on the fogs rise higher and higher, and 

 finally a day comes when the summit is covered. This is the precursor 



