4 OBSERVATIONS ON I. 



At several of the above places, the stations were not strictly identical ; and it 

 may be seen, from the results at Washington, what changes are often produced by a 

 very short distance, such as from the Capitol to the Observatory ; still, even here, 

 Capt. Lefroy's results and my own both confirm the fact that the difference exists. 

 It will be seen, also, that there is a discrepancy of 0.011 between us at New York, 

 and likewise one of 0.009 at Philadelphia; but even that amount of difference 

 will be found by the same observer, with the same instruments, at different times, 

 in an identical locality. 



Reduction of my Observations to the Arbitrary Scale most commonly used. — In the 

 following tables, I have assumed the terrestrial magnetism at Longworth's Garden, 

 in Cincinnati, as a provisional standard of comparison, or a base of all of my observa- 

 tions. It was, therefore, important that this standard or base should be strictly 

 compared with other bases more generally connected and compared; and it was 

 with this view that, at the request of Col. Sabine, I made observations at Toronto 

 the base of the British observations in North America. 



Col. Sabine has made the comparison as extensively as possible from the obser- 

 vations of Capt. Lefroy and myself; and his conclusions are quoted below, from the 

 paper to which I have already referred, p. 313. 



" Collecting into one view the results of the three comparisons, we have — 



"1. By the direct comparison of the horizontal force at Cincinnati and Toronto, 

 by Dr. Locke, 1.795.^ 



"2. By three intermediate stations, at which the ratios of the horizontal force 

 Avere determined by Dr. Lockk to the force at Cincinnati, and by Capt. Lefroy to 

 the force at Toronto, 1.794. 



"3. By six intermediate stations at which the ratios of the horizontal force 

 were determined to the force at Cincinnati by Dr. Locke, and the ratios of the total 

 force to its value at Toronto by Capt. Lefrot, 1.796. 



" The total force at Cincinnati, the base station of Dr. Locke's survey, has, there- 

 fore, been taken at 1.795." 



Hence, to reduce the total intensity at any station named in my paper to the 

 comparative or " arbitrary scale," it is only necessary to apply the following simple 

 equation — 



_ 1.795 a 

 ^^ 1000 ' 

 in which a = the tabulated total intensity at any station in my papers, and x = the 

 arbitrary or comparative quantity sought. 



Explanation of ilie following Tables. — At the head of each is placed the date, the 

 name of the place, the latitude and the longitude, either from the best observations 

 or from the most approved maps — Tanner's being, for the most part, preferred. In 

 the Iowa and Wisconsin region, Jackson's map was used, and in the region of Lake 

 Superior, Capt. Bayfield's, as published by the Society for the Promotion of Useful 

 Knowledge. In regions where nothing better was available, I have reduced the 

 United States surveys, made by chain-measuring in the woods, to an approximate 



Total force at Cincinnati. 



