347 



CONTKIBUTIONS TO GEOGRAPHY, No. I. 



On the Latitude and Longitude of Chicago, Illinois. By Lieut. 

 Colonel J. D. Graham, of the U. S. Corps of Topographical 



Engineers. 



Chicago, Illinois, November 2'dth, 1858. 



To the American Philosopliical Society, Philadelphia. 



In my letter of the 2d of April last, published at pp. 313 to 315 in 

 Vol. 6 (No. 59) of the proceedings of the Am. Phil. Society, I stated 

 that I was induced, in the month of January previous, to make a few 

 observations with my sextant of 7^ inches radius, artificial horizon of 

 quicksilver, and chronometer, for the approximate determination of the 

 latitude of the City Hall, or Court House of Chicago. The approx- 

 imate result announced for the latitude of that point, was 41° 53' 

 09".7 N. 



It was derived from two nights' observations, rather hastily made, 

 consisting in all of 19 observed double altitudes of Polaris {<x. Ursse 

 Minoris), north, and 13 observed double (circum-meridian) altitudes 

 of /3 Orionis, south. 



Desiring to obtain a closer approximation to the true latitude of the 

 point in question, and having no other instruments at my command 

 than those already described, I made, with them, on the nights of the 

 15th and 23d of August, and the ISth of September of the present 

 year, the observations given below. 



The stars observed on were arranged in pairs, and were selected 

 so that one of each pair should pass the meridian to the north and the 

 other to the south of the zenith, and at nearly equal meridian alti- 

 tudes. This was done for the purpose of eliminating any small and 

 imperceptible errors that might appertain to the sextant, either in the 

 graduation of the limb, or circular arc — in the centering of the same — 

 in the adjustment of the mirrors — in the collimation of the telescope, 

 or in the refraction as affected by any peculiar state of the atmosphere, 

 especially in cases where neither thermometer nor barometer is at 

 hand to aid in this last correction; — and also for the purpose of elimi- 

 nating any error of personal equation appertaining to the observer. 

 The index error, as it is called, of a sextant or other reflecting instru- 

 ment, can only be approximately ascertained. Any residual error in 

 its measurement is also eliminated by the arrangement above mentioned. 



The time stars were also selected in pairs and of very nearly the 

 same declination — both of north declination in this latitude — and of 

 such right ascensions that one could be observed when near the east 



