MOUNTAIN OBSERVATORIES IN AMERICA AND EUROPE. 1 



At the Lick Observatory both the advantages named above are se- 

 cured ; that is, increased transparency and greatly increased steadiness. 

 The astronomical observations made on the Santis show the same to 

 be sometimes true at this station, though both advantages are rarely 

 secured at high mountain-stations. 



In astronomical observations it is desirable that the image of the 

 star under examination should be as bright as possible ; and as steady 

 as possible — as free from twinkling, as has been said. Of the two 

 requirements the second is far more important for all observations in 

 which accurate measures of the positions of stars are needed ; and in 

 most spectroscopic observations. 



A striking demonstration of the relative importance of the two factors 

 is often afforded during the times when a fog is slowly forming in the 

 atmosphere. While the air is perfectly clear, both components of a 

 double star, for example, seen through the telescope, will appear quite 

 brilliant, but each component will be so unsteady (will twinkle so) 

 that measures of their distance apart will be difficult to make. As the 

 fog comes on, its effect is to equalize the temperatures of different 

 layers of the atmosphere and thus to increase the steadiness of the star. 

 At the same time another effect of the fog is to absorb some of the 

 star's light and thus to decrease its brilliancy. 



If the fog forms slowly, it is instructive to watch the gradual changes 

 in brilliancy and to note how little effect these changes have upon the 

 measures ; and to remark, on the other hand, the great increase of pre- 

 cision in the measurements due to the increased steadiness of the 

 star-images. 



A transparent air is very desirable ; a steady air is essential for most 

 astroDomical work. 



The conditions which produce steady seeing depend, in general, 

 upon the arrangement of the layers of atmosphere above the observing 

 station. If we imagine the observer to be situated on an extensive 

 level plain, as on the steppes of Eussia, a small island in the tropical 

 ocean, or the plains of Lombardy, and if the air is quite still, the sepa- 

 rate layers of the atmosphere will be arranged in strata parallel to the 

 earth's surface. The lowest stratum will be the warmest, the highest 

 the least warm. The transition from the temperature of one stratum to 

 that of the next will be gradual and regular. The changes of moisture 

 and of density in the various strata will be gradual and not abrupt. 



A ray of light from a star falling on such a series of strata will pass 

 through them all in a regular smooth curve. In the telescope no 

 twinkling of the star-image will be noticed. 



Now if some of these strata are very cold, while the adjacent ones 

 are warm, the atmosphere in such regions will be in rapid and irregu- 



