170 EXPLORATIONS ACROSS THE GREAT BASIN OF UTAH. 



other facilities offered to me by the distinguished Secretary of the Institution, Prof. 

 Joseph Henry. 



I am, sir, your obedient servant, 



Henry Engelmann. 

 Capt. J. H. Simpson, 



Tojiof/raphiral Engineers, U. S. A., 



In charye of Exjilnriity Expedition. 



INSTRUMENTS. 



On starting from Fort Leavenworth, we were provided with three cistern barom- 

 eters, Nos. 1062, 1237, and 1279, made by James Green, of New York, with scales 

 graduated down to 20 inches, and with verniers reading to thousandths of an inch. 

 These instruments, as improved now by Mr. Green, were again found to be admirably 

 adapted to the wants of exploring- expeditions, when they are transported over many 

 hundreds of miles of rough mountain roads, and exposed to all accidents contingent to 

 their daily use on the road and in camp. One of their principal advantages is the 

 readiness with which they may he repaired in the held when damaged by long use or 

 broken by accidents, which will happen to the most careful observer. Against such 

 emergencies we were provided with several glass tubes, pure mercury, and other 

 requisites. A portable tripod was furnished by Mr. Green with the instruments, and 

 found very useful, indeed, indispensable. The immovable support which it gives can 

 often not be obtained otherwise in the fi'eld, and adds to the correctness of the observa- 

 tions and to the preservation of the instruments. 



We were also provided with aneroid barometers, which, however, were not used, 

 as no reliable results could be expected from them at the elevation and in the climate 

 where Ave might have needed them most. Besides these, we had a number of ther- 

 mometers and a rain-gauge. 



OBSERVATIONS. 



Regular observations of the barometer, dry and wet bulb thermometers, cloudi- 

 ness of the sky, direction and force of the wind, quantity of rain, &c, were kept up 

 from the time of the arrival of the party at Fort Leavenworth, Ka'ns., in May, 

 1858, to our return there in October, 1859. As it was desirable to obtain observa- 

 tions for as long a period as possible at each successive camp, and from the warmest 

 to the coldest time of the day, the first observation was made soon after reaching a 

 camp, and the last one shortly before leaving it again, conforming, as much as possible, 

 to the hours of 6 a. m., 9 a. m., 12 m., 3 p. m., 6 p. m., and 9 p. m., which were fixed 

 upon as the regular hours for observations when in camp. In the mountain 

 regions, during the explorations between Camp Floyd, Utah, and Fort Bridger, Utah, 

 and between Cam}) Floyd and Carson Valley, Utah, numerous observations were 

 made on the road, with a view to the construction of the profile. Besides, a 

 very large number of hourly observations were most carefully made at every point 

 where a protracted stay offered an opportunity, in order to obtain data for the 

 determination of the daily variation of the atmospheric pressure, the temperature, 

 the elastic force and weight of vapor, and the relative humidity of the different dis- 



