Variation and Dip of the Magnetic Needle. 307 



bad observations, or were very much influenced, by local attrac- 

 tion. This chart can of course lay claim to no greater accuracy 

 than that to which the individual observations are entitled ; yet 

 it has the advantage of presenting to the eye at one view a dis- 

 tinct summary of all the observations, and moreover it shows at 

 a glance the approximate variation at places where observations 

 have never been taken. It is of course desirable that the chart 

 should be verified as extensively as may be, and if it should be 

 the means of stimulating a single individual to undertake a series 

 of accurate magnetic observations, taking care to present them to 

 the public, the labor which this work has cost me will not have 

 been expended altogether in vain. 



I had originally contemplated merely a collection of observa- 

 tions on the variation of the needle. In the course of my in- 

 quiries, however, I met with a few observations of the dip, and 

 concluded to unite them all in the same article. All which I have 

 been able to collect are contained in the following table. Those 

 made by Professors Bache and Courtenay are from the Transac- 

 tions of the American Philosophical Society, Vol. V. New Series ; 

 those made at Cambridge are from the Memoirs of the American 

 Academy, Vol, I. p. 68 ; those made by Sir John Franklin, Cap- 

 tains Sabine and Back, are given in the Philosophical Transac- 

 tions of London ; the observation at Charlottesville was commu- 

 nicated in a letter by Prof. Patterson ; those in Ohio were received 

 from Prof. Locke, of Cincinnati ; and the remainder are from 

 Long's Expedition to the Rocky Mountains in 1819 and 1820, 

 and were furnished me by Mr. HerricJc. 



