GEOLOGICAL SURVEY OF THE TEREITOEIES. 



797 



The magnetic variation was determiued by observing the azimuth of 

 Polaris with the gradienter, an instrument reading to minutes ; the re- 

 sults are to be depended on to that degree of precision. 1 append the 

 list: 



Date. 



Place of observation. 



Fort Ellis 



White Mountain Hot Spriug.s 



Yellowstoue Falls 



Lower Geyser Basin 



Jlonth of Indian Creek 



Virginia City * 



Gallatin City 



Head of Gallatin, (latitude 45° 15') 

 Helena 



19 59 



Given me by Mr. Corbett, of Virginia City, United States Dop. Min. Sur. 



The instruments placed at my disposal for hypsometric and meteoro- 

 logical work were six of Green's cistern barometers, two of these being 

 single and four of them double, vernier barometers, with a sufScieut" 

 supply of wet and dry bulb and maximum and minimum thermometers, 

 and a good supply of aneroids. 



I decided upon Fort Hall, Idaho, and Virginia City, Montana, as 

 suitable and available base-stations. The altitude of the former place 

 was determined by coincident observations at this point, and Ogden, 

 Utah, (the elevation of which was taken from raih'oad levels,) for three 

 weeks during the month of June; that of Virginia City, by coincident 

 observations between that point and Fort Hall, during the months of 

 Jane, July, August, and September. 



The base observations at Fort Hall were made by Mr. Wm. oS^ichol- 

 son, of the survey. They were made hourly ^rom 7 a. m. to 9 p. m., 

 during the whole time that the parties were in the field. 



The United States signal-service, through its sergeant observer at 

 Virginia City, Mr. A. B. Knight, kindly allowed me the use of tlie obser- 

 vations taken there, for hypsometric purposes, modifying slightly, to 

 suit my wishes, their method of observation. One of the single vernier 

 barometers, with dry and wet bulb and maximum and minimum ther- 

 mometers, ^vas left at Fort Hall, for Mr. Nicholson's use. I gave to 

 Mr. Hering, who took charge of this work in the Snake Eiver division, 

 two of them, with the other necessary instruments, and reserved the 

 others for use in the Yellowstone division. 



Aneroids have been used considerably, especially where closely accu- 

 rate results were not required, and have been found, if compared daily 

 with the mercurial barometers, and carefully used, to be swfl&cieutly 

 accurate for the purposes for which they were employed. 



I have paid some attention to elevations of the water-level of streams, 

 not only for getting the rate of fall, which may be of use for purposes 

 of irrigation, &c., but as indicating vvhat may be called the water-con- 

 tour of the country, which, certainly exi^resses the general elevation of 

 the country better than any other class of observations. 



As the question of the location of a railroad from some point on the 

 Central Pacific llailroad to the settled portions of Montana is now of 



