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CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 





-J%^ 



When the steadiness of the atmosphere begins to break, 

 the images of the Sun, Moon and planets exhibit occasional 

 tremors or shiverings: and even when this vibratory move- 

 ment has become nearly continuous, but not rapid, the 

 actual and apparently more condensed outline of the image 

 can be selected for measures of precision in moments of 

 quietude. As the unsteadiness of the atmosphere increases, 

 the vibration of the limbs increases in rapidity and ampli- 

 tude until there is finally created a blurred outline of Sun, 

 Moon or planet, which necessarily presents an increased 

 diameter of the object. The stars are similarly affected; 

 with the first signs of unsteadiness in the atmosphere the 

 diffraction rings are broken, and disappear as the unsteadi- 

 ness increases; the spurious disc is broken into pieces; its 

 march is peculiar, in presenting one fragment after another 

 with more or less frequency, but may continue with a waltz- 

 ing movement until the unsteadiness has increased and the 

 spurious disc is without form, dilated, jumping wildly in 

 every direction, and a wretched object for observation. 



Similar conditions are exhibited by geodetic signals of 

 all classes. In a serene atmosphere poles are seen very 

 steady as fine, sharp images, sometimes finer than the 

 micrometer threads. When this atmosphere is disturbed, 

 tremors or shiverings carry the images to right or left one 

 or more diameters, and the observer selects moments of 

 quietude for first-class observations; finally, as the unsteadi- 

 ness of the atmosphere increases, the images become large, 

 confused, blurred and faint, and observations are difficult to 

 make and unsatisfactory. In the quiet atmosphere, white 

 poles five feet long and four and six-tenths inches in diameter, 

 projected upon a dark background, have been seen with the 

 naked eye at distances of four and one-half miles across 

 water. In times of great unsteadiness and equally clear 

 atmosphere they are difficult to see in the telescope. 



Heliotrope images behave precisely as those of the stars. 

 On a certain line of the main triangulation of the Pacific 

 Coast, where plains, several mountain ridges and inter- 

 vening valleys were crossed by the line of sight, and where 



