Observations on Surveying Instruments. 273 



scientific value, and nfay be made at any place in a variety 

 of ways, with far less trouble than their importance might 

 demand.* 



In October, 1822, in company with I. Mullett, Esq., survey- 

 or general of Michigan, I made several observations, to de- 

 termine the precise variation of the needle at Detroit, and 

 the mean of the different observations was 3° 1 3' 22" declin- 

 ation eastwardly. I have, during the present month, repeated 

 the observations in company with the same gentleman, and 

 find 2° 50' variation eastwardly ; thus showing a traverse of 

 the magnetic meridian to the westward of 23' 22" in little 

 more than five years, or about 4' T 4 „ per year. On the Island 

 of Michilimackinac in July last, I found the variation east- 

 wardly to be 2° 59'. In the winter of 1825-6, on the Grand 

 River of Lake Michigan, in the western part of the peninsula, 

 I observed the variation every clear night, for several months, 

 and every night in a different place, extending over a tract 

 of country about seventy miles long by twenty-four broad. 

 The results were various, from 3° 45' E., to 6° E,, varying 

 sometimes 50' in a distance of six miles ; and that without 

 any apparent cause, as the country is entirely alluvial, and 

 scarely a trace of iron can be found. The declination, how- 

 ever, generally appeared to increase in going westwardly. 



Practising surveyors, in all parts of the United States, 

 whether amongst the primitive mountains, or on the alluvial 

 plains, often complain of the errors and perplexities arising 

 from the aberrations of the needle, and suppose the cause, 

 which they call local attraction, to exist in the earth. 



In hilly countries, where ores abound, it is reasonable to 

 suppose that they may exert an influence upon the needle, 

 and sometimes cause it to vary from its general direction, 

 but on extensive alluvial plains, where there is no trace of ores 

 of any kind, its aberrations are, on this supposition, inexplica- 

 ble ; and I am well convinced that in most cases they de- 

 pend entirely on another cause, which I do not recollect to 

 have seen mentioned by any writer on the subject, namely, 

 the developement of electricity or magnetism, or both, in 

 some part of the glass of the compass, by its friction against 

 the clothes of the person carrying the instrument. 



* Mr. Mullett keeps a register of the daily variation at Detroit. 



Vol. XIV.— No. 2. 9 



