F. GEOGRAPHY. 239 



connected systems of trigonometric leveling have been carried 

 over the whole area surveyed, and the check observations are 

 so arranged that the probable error can be easily determined, 

 and it is hoped that the system will prove accurate enough 

 to throw some light on the amount of refraction at great ele- 

 vations. By these methods the altitudes of many high points 

 have been determined, from which to construct a map in con- 

 tours 200 vertical feet apart, on a scale of two miles to one 

 inch. 4 D, 1873,11,274. 



GEODESY IN THE COLORADO RIVER. 



The report of the explorations of Professor Powell in the 

 caiions of the Colorado River impresses one with the very 

 careful manner in which he has attempted to carry out the 

 work undertaken by him. Although only small portable in- 

 struments have necessarily been used, it would seem that re- 

 sults of a very high degree of approximation, not to say ac- 

 curacy, will be attained. To a geodesist the most interesting- 

 portion of his report details the method of surveying the dis- 

 trict of the Grand Caiion. A base-line was measured with 

 wooden rods leveled on trestles, and aligned by means of a 

 transit-instrument between the extremities of the line. The 

 latitude and longitude of the northern extremity of the base- 

 line were accurately determined. Three rods were used, al- 

 ways remaining in position, as protection against accidental 

 movement. The base-line was found to be 48,100 feet in 

 length, and, starting from its extremities, a system of trian- 

 gles was expanded so as to embrace all the country immedi- 

 ately inclosing the Colorado River. The angles of these 

 triangles were measured with a seven-inch theodolite. Their 

 sides were often twenty-five to thirty miles long, and occa- 

 sionally much longer. Six of the more distant points in these 

 triangles were occupied as astronomical stations, and, based 

 on this primary triangulation, a vast net-work of secondary 

 triangles was constructed. The second summer of work in 

 this region was occupied by descending through the canons 

 in boats, and fixing the course of the river and the topograph- 

 ical features of the walls of the caiion, in which latter work 

 the compass, gradienter, and barometer were in continual 

 requisition. An area of about 45,000 square miles has been 

 already surveyed by Professor Powell's parties, more than 



