MOUNTAIN OBSERVATORIES IN AMERICA AND EUROPE. 51 



ing to note how well the native vegetation . . . would have indicated to the 

 botanist the relative immunity of this mountain-top from fogs and long-continued 

 rains. 



Professor Grreene finds that the species of trees, etc., on Mount 

 Hamilton are nearly always those of the dry interior of the State rather 

 than those of the coast ranges, and that the botanical region has its 

 affinities with the dry San Bernardino mountains rather than with the 

 peaks of the coast range lying very much nearer to it. 



Purity of the atmosphere at Mt. Hamilton: North winds, in summer, 

 bring dust from the Sacramento valley to surround us, and forest-fires 

 near us, or even those of Oregon, sometimes fill the whole atmosphere 

 with haze. When neither of these hindrances is obvious, that is for 

 a very large proportion of the days, the air is of great purity. Two 

 obvious proofs of it may be cited. The sky -glare near the sun is 

 weak ; and the atmospheric spectrum at sunset is beautifully clear and 

 finely graded. Nearly all our summer sunsets are cloudless. The sky 

 is a deep orange near the horizon and shades off through the prismatic 

 colors, in a vertical circle, to the pure upper blue. No one who has 

 seen this effect once will forget it, and to mention it will recall it to all 

 who have lived with us. The photographs of sunsets at Mount Hamil- 

 ton made by Mr. Colton * show the sun sinking into banks of dust or 

 fog, not into clouds (consult the accompanying table of zenith-distances) 



Daytime observations : All our experience at Mount Hamilton goes 

 to show that the steadiness of vision in the daj^time is certainly no 

 better than that of American observatories generally, and that it is, 

 |)i'obably, somewhat less good, though the difference is not very strik- 

 ing. The reason is twofold. In the first place the topographical situ- 

 ation of the observatory, surrounded as it is on all sides by steep 

 slopes, exposes the instruments to whatever ascending currents of air 

 there may be far more than if the observatory plateau were larger. 

 This is a cause which can never be removed. 



In the second place the slopes which immediately surround the 

 plateau are composed of fragments of bare rock, which become in- 

 tensely heated during the day, and whose radiations seriously affect the 

 seeing. This cause can be done away with by planting trees and sow- 

 ing grass and vines over the rock slopes. Experiments in this direction 

 were urged on the Lick Trustees in 1881 and subsequently ; and were 

 begun in 1888. A considerable amount of water is needed in our 

 excessively dry, and long, summers to prevent ordinary grasses from 

 dying by drought, and there has been no adequate quantity of water 

 available for this purpose. The (rain-water) reservoir capacity was 

 considerably increased during the summer of 1895, and some water 



* Contributions from the Lick Observatory, No. 5. 



