a. * 
Dip and Variation of the Magnetic Needle. 93 
Art. IX.—On the Dip and Variation of the Magnetic Needle 
in the United States ; by Ettas Loomis, Professor of Mathe- 
matics and Natural Philosophy in Western Reserve College. 
el. ant 
[Communicated to the ticut Acad. of Arts and Sciences, April 28, 1842.] 
I. Dip of the Magnetic Needle. — 
I propos 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. ‘I'he 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° 41’. In 1839, I found it to be 74° 20’.1, 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° 21’; 
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 29 35’. Now I re- 
gard it quite certain, that if the above numbers were obtained by 
observations in all the different 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 
