144 



ASTRONOMY AND GEODESY. 



The observations made in California were computed in the field by myself, Lieutenant 

 Whipple, and James Nooney. They were all subsequently revised by Professor Hubbard, and 

 the new element of corresponding observations introduced into the computations for longitude. 

 The observations on the Eio Bravo were computed in the field by myself and Assistants J. H. 

 Clark and J. O'Donoghue, and revised in the office by Captain George Thorn, corps of topo- 

 graphical engineers, and Assistant C. K. Thom. 



The observations on the parallels 31° 41' and 31° 20' were all computed in the field by Prin- 

 cipal Assistant J. H. Clark, Lieutenant Turnbull, corps of topographical engineers, and As- 



offi 



mad 



Michler 



A. 



Determination of the line ft 



i 



Mexico, from tlie Initial Point on ilie Pacific ocean to tie jMint where the " Gila river 



empties into the Colorado." By Brevet Major William H. Emory, Chief 



om 



The 



marine league south oi tne port oi ban -Uiego, t( 



most obvious way of determining the direction of this line was to connect the two points by 

 triangulation, and in this way ascertain their relative positions on the face of the earth, and 

 compute the azimuth of the line joining them. But the character of the intervening country 

 made it impossible to pursue this mode of operating when the time and means at the disposal of 

 the joint commission were considered. Triangulation is the surest^ but it is the slowest and 

 most expensive method of surveying, even in old-settled countries, where the stations to be 

 selected are easily accessible in wagons. In the country under consideration obstacles pre- 

 sented themselves almost insurmountable. The whole distance, about 148 miles, may be 

 divided into two nearly equal parts, differing in character, but both equally unfavorable to 

 geodetic operations. The first, rising in steppes 

 tation, attains, in abrupt ascents, the height of five or six thousand feet in the short space of 



m 



tliirty miles. From this point, for about thirty miles, the country is occupied by a succession 

 of parallel ridges, striking the boundary nearly at right angles, and separated by deep, and 

 sometimes impassable chasms. It then falls abruptly to near the level of the sea. Ihe 

 remainder of the line stretches across the desert of shifting sand at the head of the Gulf of Cali- 

 fornia, destitute, for the most part, of both water and vegetation. 



Tlio fnllnxirinor is \\\a c\rA(^r in wTiipTi orn nrronorprl fliP cnTiiPpfs Pmbranftd in the detcrminatlOfl 



from 



of the line : 



1. The longitude of Camp Riley, near the Initial Point. 



2. The longitude of Carap near the junction of the Gila and Colorado. 



3. The latitude of Camp Riley, near the Initial Point. 



4. The latitude of Camp near the junction of the Gila and Colorado, 



5. Transfer of the latitude and longitude of Camp Riley, by triangulation, to the Initial 

 Point. 



6. Azimuth of straight line from Initial Point, on Pacific, to junction of Gila and Colorado. 



