96 CALIFORNIA ACADEMY OF SCIENCES. IProc. 3D Ser. 



Precision Observations at Low Elevations. 



And yet there are climatic conditions at comparatively 

 low elevations, which are supremely favorable for observa- 

 tions of precision at night. The experience of the observ- 

 ers of the United States Coast and Geodetic Survey on 

 the immense, elevated, arid plains from El Paso to San 

 Diego is very instructive. During the hot cloudless days 

 the atmosphere over the immediate surface of the parched 

 earth was in violent unsteadiness, and the heliotrope images 

 at even short distances flared out wildly like burning 

 houses. They were the worst possible objects for observa- 

 tions of precision. After sunset the conditions were sud- 

 denly changed. With a cloudless sky and a minimum of 

 aqueous vapor in the attenuated air (Barometer about 26 

 inches) radiation was quickly effective. The temperature 

 fell many degrees in a short time, and the air became 

 supremely quiet. The latitude telescope showed stars as 

 minute discs with diffraction rings running along the 

 micrometer thread with such extreme steadiness that it was 

 impossible to be in doubt more than one-tenth of a second 

 of arc in measurement; and in the transit instrument the 

 star marched with absolute regularity across the reticule. 

 It was an experience that an observer of over thirty years 

 in the field declared he had never before enjoyed. 



Per contra, on the low Yolo plains of the Sacramento 

 Valley in California, we have had the azimuth signal-light 

 (distance eleven miles), in a calm night, running wildly up 

 and down the vertical thread of the theodolite through five 

 minutes of arc, in an ever changing line of broken stars of 

 the prismatic colors, and yet showing little or no horizontal 

 motion. The range of height of this column of images was 

 eighty-seven feet. 



In the great Gangetic plains of India the night signals of 

 the triangulation parties, when shown from towers of fifty 

 feet elevation, frequently appeared as continuous columns 

 of light sixty feet high. 



In the triangulation of the western coast of Lower 

 California by the United States Steamer "Thetis," the 



