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CARNEGIE INSTITUTION OP WASHINGTON. 



improves and reaches a maximum, where it remains for some time. 

 The effect of the heating of the mountain then becomes apparent 

 and the definition deteriorates. The disturbances at the sun's limb 

 under these conditions do not resemble those seen immediately after 

 sunrise, but have a fluttering appearance, which we are accustomed 

 to speak of as the "heating effect." In the late afternoon the seeing 

 usually improves, but it is rarely very good at midday. This is not 

 a rule without exceptions, however, as we have sometimes recorded 

 nearly perfect definition during the hottest hours of the day. 



Everyone who has noted the heated air above the surface of the 

 ground will wonder, in considering the effect of such disturbances 

 upon solar observations, whether these disturbances rise to a great 

 height. A casual observation is sufficient to show that the dis- 

 turbance decreases rapidly in passing upward from the ground, 

 but it is, of course, quite impossible to determine by means of 

 the unaided eye the probable effect of this disturbance on telescopic 

 observations. We have accordingly made many observations of 

 the sun with the 3 ^4 -inch telescope supported in a pine tree at 

 heig'hts above the ground ranging from 20 to 80 feet. The results of 

 these observations clearly indicate that a telescope employed in solar 

 work should be mounted as high above the ground as circumstances 

 warrant. At the lower elevations in the tree the advantage over posi- 

 tions still nearer to the ground was sometimes not appreciable ; but 

 at a height of 80 feet above the ground the improvement in definition 

 was very distinct. Probably this is one of the reasons why the solar 

 definition with the 40-inch Yerkes telescope averages considerably 

 better than we expected it would, for with this telescope the object- 

 glass is over 70 feet above the ground. 



Seeing. 



♦Rain. 



t Snow. 



