50 



sary to reduce ineanderliaes by the ordinary process of dead-reckouing, individual judgment for 

 the percentage to be allowed being suiBcient to reduce to the points astronomically determined. 



The odometers were attached to a little two-wheeled vehicle constructed for the purpose, which 

 was talien charge of by a soldier whose sole duty was to keep account of the distances measured. 

 There are difficulties connected with the mechanical contrivances of the odometer. 1st. The leather 

 covering, however carefully made, will not keep the dust from working into the interior of the 

 instrument on account of the manner in which the frame holding the circles is introduced. 2d. The 

 circles themselves work loose from each other for the want of two nuts, one with a right-handed 

 screwand the other with a left-handed one, at the back of these plates, for fastening them together. 



METEOROLOGICAL OBSERVATIONS. 



INSTRUMENTS USED. 



During the field-season cistern barometers, ISTos. 1378, 1555, and 1566, made by James Green, 

 New York ; aneroids, Nos. 22 and 37, furnished by Charles G. Ewing, optician, San Francisco ; 

 and hygrometers, Nos. 1631 and 2348, by Green, were used. Thermo-barometers, Nos. 1 and 2, by 

 Green, were carried to the field, but beyond comparing their indications with those of the cisterns 

 at Elko, Camp Ruby, Hamilton, Ice Creek, and Cave Valley, no use was made of them for 

 hypsometrical purposes. 



OBSERVATIONS IN THE FIELD. 



Ilourly observations were taken at Camp Halleck from June 16 to June 29, inclusive, and 

 at Camp Ruby, Hamilton, and West Point, over intervals of from five to eight days, for the pur- 

 pose of securing tables of horary corrections to be applied to observations for hypsometrical 

 purposes. On the march tri-daily observations were made at all camps of a day or more, and, at 

 camps for one night only, at 7 a. m. and 9 p. m. 



The anei'oids were used only in connection with the odometer for securing an approximate 

 profile of the route between camps, the altitudes of which latter were deduced from cistern-barome- 

 ter observations. 



These observations have all been reduced and computed, and tlie results appear on the map of 

 the reconnaissance. 



COMPARISONS OE BAROMETERS, ETC. 



Before taking the field, the barometers and attached thermometers were carefully compared 

 with Green's standard cisiern-barometei', No. 1571, in Colonel Williamson's ottice in San Francisco, 

 and their relative and absohite errors deduced. These comparisons extended over an interval of 

 nine days, from June 2 to .June 11, 1869, and isicluded cistern barometers Nos. 1566, 1378, 1555, and 

 1282, and aneroids 22 and 37. 



At intervals during the season frequent com[)aris;>ns were made to check changes in the zero 

 of the scales of the various iustruments; at Elko, Nev., from June 29 to July 3, fifteen com- 

 parisons; at Camp Ruby, from July 9 to July 12, twehe comi)arisous ; at Hamilton, from July 

 16 to July 21, seventeen comparisons ; at Cave Valley, from August 9 to August 12, nine 

 comparisons; at West Point, from September 23 to September 27, fourteen comparisons ; at Las 

 Vegas, from October 1 to October 12, sixteen comparisons; at Indian Springs, from October 

 26 to October 30, thirteen comparisons ; and at the close of the field-season the instruments 

 were again compared with standard 1571 at San Francisco. These comparisons in the case of 

 cistern-barometers gave very tav(U'able results, showing but very slight changes in their relative 

 errors from transportation, but the aneroids, being mechanical devices, suffered considerable 

 shiftings of parts and consciiuent changes in their index errors, other than those due to tempera- 

 ture or from want of coni[)ensatiou. The extreme variation in errors throughout the season being 

 from +".010 to ".774 for aneroid 22, and from — 0".034 to — 0".186 for aneroid 37, but since these 

 changes a;ippar fiom the comparisons to have been gradual, very good results were derived from 

 the aneroid work. 



