Dip and Variatmi of the Magnetic Needle. 93 



Art. IX. — On the Dip and Variation of the Magnetic Needle 

 in the United States ; by Elias Loomis, Professor of Mathe- 

 matics and Natural Philosophy in Western Reserve College. 



[Communicated to the Connecticut Acad, of Arts and Sciences, April 28, 1842.] 



I. Dip of the Magnetic Needle. 



I PROPOSE in the present article to discuss all the observations 

 of magnetic dip in the United States, which have come to my 

 knowledge. Most of these have been given in former numbers 

 of this Journal, and others will be found embodied in the tables 

 which follow. In presenting this enquiry, it is important accu- 

 rately to determine the annual change of dip. Unfortunately, the 

 materials for this purpose are quite too scanty, and the results 

 very discordant. The earliest observations of the kind I have 

 met with, were made by Prof Williams, at Cambridge, Mass. 

 1780-83, and published in the Memoirs of the American Acad- 

 emy, Vol. I, p. 68. According to this authority, the dip in 1783 

 was 69° 4P. In 1839, I found it to be 74° 20M, indicating an 

 increase of five minutes per year. This result might be received 

 as worthy of confidence, were it not that a remark of Prof Wil- 

 liams throws a suspicion over the accuracy of his observations. 

 He says : " the dip is subject to rather greater diurnal alterations 

 than the variation ; but they do not seem to be so regular in 

 their changes. The least dip I have ever observed, was 68° 2V ; 

 the greatest, 70° 56'." According to observations at Milan, the 

 diurnal change of dip is somewhat above one minute in summer, 

 and about half a minute in winter. The diurnal change of the 

 variation is, for this latitude, in summer, nearly fifteen minutes. 

 The apparent diurnal motion observed by Prof Williams must 

 then have been due to the inaccuracy of his observations. The 

 entire range of his observations he reports at 2° 35'. Now I re- 

 gard it quite certain, that if the above numbers were obtained by 

 observations in all the difi'erent positions of the needle as at pres- 

 ent practiced, then his instrument was utterly worthless. The 

 probability is that the necessary reversals were entirely neglected, 

 and as no mention is made of reversing the poles of the needle, 

 the presumption is that it was not attended to. In this case, the 

 observations would be charged with a constant error whose 



