Dip and Variation of the Magnetic Needle. 95 
These equations solved by the method of minimum squares give 
d=1.0801, c=+0.00987, y= + 0.00303, and the direction of the 
isoclinal lines is from N. 72° 56’ W. to S. 72° 56’ E. Computing 
from these data the dip at the several stations, we obtain the differ- 
ences given in the sixth column above. a Cote sans Dessein; pte 
difference between the ob dand 63/.9. 
An error of one degree may perhaps have been conimitted:i in re- 
cording or transcribing this observation. At all events this ob- 
servation seems entitled to less weight than the others, and we 
shall perhaps do best to reject it altogether. The remaining 
equations solved as before, furnish us 6 =.97301, c=+0.01301, 
y=+0.00397. Computing again the dip at the several stations, 
we obtain the differences given in the last column above. The 
mean dip at St. Louis for 1819 was then, according to these ob- 
servations, 69° 59’.8. The-dip in 1840, according to p. 102, was 
69° 20’.7. Decrease 39’.1 in twenty one years, that is, 1/.9 per 
ear. This result would be deserving of confidence if the ob- 
servations of 1819 could be depended upon. For information on 
this point, I wrote to Major Graham, who made the observations 
in question, from whom I learned that the poles of the needle 
employed were never reversed; and that he knew of no data 
for determining the correction necessary for this omission. Here 
then is a constant error of unknown amount, which renders the 
observations quite useless for our present purpose. ‘This is a re- 
sult much to be regretted; yet if no observations were made at 
the time with the polarity of the needle reversed, the loss seems 
; for if the instrument were still prevetved and in good 
cinditiens, we obviously could not assume the inequality of the 
arins of the needle to be the same as in 1819. I do not find then 
any observations of dip made in this country before 1822, which 
are capable of informing us whether the dip has been increasing 
or diminishing. 
The next observations in respect of age were made at New 
York city. Here we have five observations. In order to deduce 
from them the most probable results, let us put 4 for the mean 
dip, Jan. 1, 1822, and 4 for the annual motion; we thus obtain 
five equations of condition. 
