1G2 EXPLORATIONS ACROSS THE GREAT BASIN OF UTAH. 



has the advantage of securing uniformity of result, an avoidance of error in the hasty 

 reading of the instrument and record of the angles, and a general satisfaction in the 

 observations. Of course in'the case of only occasional glimpses of the sun on account of 

 intervening clouds this mode should not be practiced. 



Lieutenant Vatman Huhmlts n salts of (djsri rations for latitude and longitude. 



Washington, D. C, March 1, 18G0. 



Sir: The subjoined table gives the geographical positions for the most important 

 points on the new routes between Fort Bridger and Genoa, Utah. 



It will be observed that the longitude of Camp Floyd, and consequently chrono- 

 metrically, that of Great Salt Lake City, has been decreased about 11/ from that given 

 by Colonel Fremont. This change, however, has been made only when a careful series 

 of observations on the moon and moon-culminating stars wan-anted the alteration. 

 These observations, consisting of five complete sets, were made during two lunations 

 (in the months of March and April, 1859), and a mean of all the results, which did 

 not differ essentially, was taken as the true longitude. 



The longitude of Genoa is determined from a single set of observations of the 

 same kind as the foregoing; the age of the moon and other circumstances, made it 

 impossible to take as full a series as was desirable. Between Camp Floyd and Genoa 

 other observations on the moon and moon-culminating stars, and of lunar distance 

 were made for absolute determinations of longitude. 



Equal altitudes of the sun, or double altitudes of "east and west stars," were taken 

 at intervals which, with the known error and rate of the chronometer, affords the means 

 for arriving at the longitude of intermediate points. 



Latitude has been computed from double altitudes of the sun or Polaris at nearly 

 every camp on the route. 



The computations for latitude and time have been made by myself, assisted by 

 Lieut. C. H. Collins, Topographical Engineers, and Mr. J. R. P. Mechlin; each com- 

 putation being made by two persons to guard against mistakes. The longitudes by 

 the moon and moon- culminating stars, and by lunar distances, have been computed by 

 Mr. D. G. Major of Washington. 



The instruments employed in the field were: 



1 st. A portable transit, made by Wiedemann ; focal length, two feet. After reach- 

 ing Camp Floyd, the spider-lines of the reticle were found broken; they were replaced 

 by such substitutes as could be obtained there, and it is believed the results are worthy 

 of full confidence. 



2d. One box, mean solar, chronometer by Parkinson and Frodsham, London, No. 

 1821, and two pocket chronometers, one, No. 221, by Frodsham, and one No. 8189, by 

 A. P. Walsh, London. Of these, No. 1821 was used in most cases, and a proof of its 

 reliability is to be found in the correspondence between the longitude as given by it, and 

 that determined absolutely. 



For instance, at the North Bend of Walker's River, the chronometer gives longi- 

 tude 118° 56' OS" west from Greenwich, and an observation on the moon, Alpha Vir- 



