MOUNTAIN OBSERVATORIES IN AMERICA AND EUROPE. 17 



produced by reflection from dust particles, appears to have been gen- 

 erally j)resent. 



A very interesting observation of Professor Smyth may be mentioned 

 in passing. TenerifEe is a volcanic island, and its craters were a living 

 model of corresponding formations in the moon, even to small details 

 of structure. (The same is true of Hawaii.) 



Professor Smyth refers to the uncommon dryness of the air at the 

 high stations as an "agent in producing good definition." The upper 

 air was usually dry, and very frequently strata of clouds hung on the 

 skirts of the mountain far below him. I am strongly inclined to 

 believe that these clouds materially aided good definition by confining 

 heat-waves from the mountain and sea from rising above them. Our 

 best nights at Mount Hamilton appear to be those where mists and fogs 

 cover the valley beneath us. 



The steadiness of the stars to the naked eye was frequently remarked 

 upon. They did not seem to twinkle, at first sight, though a careful 

 view showed that they did so. This simple observation is the best 

 evidence of the excellence of the observing- station. 



The high winds in the trade-wind belt were felt as a practical incon- 

 venience. Their effect upon the " seeing " is not mentioned by Pro- 

 fessor Smyth. It cannot be favorable, in general, at mountain-stations, 

 where the air in windy weather can never arrange itself in approxi- 

 mately horizontal strata. On extensive plains and at sea the effect of 

 high winds on the seeing is, on the whole, good, according to my 

 personal observation. 



The dust-haze was ever more or less present, though sometimes in 

 vastly greater quantities than at others. The "glare" near the sun 

 was directly due to this. It lay in horizontal banks. An observer at 

 the level of the sea would be more incommoded by such a bank than 

 one at a higher station if he were looking nearly vertically. But it 

 might be different for a nearly horizontal view if the higher station lay 

 in the plane of the bank. A sharp rain will always settle such dust- 

 banks, but at Mount Hamilton it almost always leaves the air unsteady. 



The transparency of the air was extremely favorable for certain 

 [physical observations, as of the zodiacal light, solar radiation meas- 

 ures, spectroscopic and polariscopic observations, etc. It is, in fact, 

 1 obvious that the mere avoidance of the lower strata of the atmosphere 

 'must be of the highest value in certain work of this kind, and that any 

 :mountain observatory will have certain advantages. Definition is im- 

 portant to the astronomer, but there are many physical problems in 

 which it plays no part. All these points, which are simple, were first 

 brought out in a clear and definite fashion by this expedition. Ten- 

 leriffe may be regarded as the parent of every mountain observatory. 



