62 Lovering and Bond on Magnetic Observations at Cambridge. 
As there can be no great difference between the variation for Bos- 
ton and Cambridge* we infer from the Table that from 1708 to 
1793 the declination diminished at the mean annual rate of 1'.8; 
and that from 1810 to 1840 it increased at the mean annual rate of 
3.6. It is probable that the change in direction took place between 
1793 and 1810 at Cambridge; whatever was the cause it does not 
appear to have affected instantaneously remote places. Dr. Bow- 
ditch who made the observations at Salem in 1805, 1808 and 1810 
supposed that the first two were smaller than they ought to be on 
account of instrumental errors. But I think that this was not the 
whole cause. The change in direction, if it had not already hap- 
pened in 1810, came soon after. Now, on the assumption that it 
was after 1810, the mean annual rate of decrease at Salem (con- 
sidering the observations at Beverly as comparable with those made 
at Salem) would be only 1.3. This is smaller than the decrease 
appears to have been on the average. I incline to think, therefore, 
that the minimum declination really occurred during the period of 
Dr. Bowditch’s observations, and that the differences which he at- 
tributed wholly to defects in the instruments were partly caused by 
this very circumstance. The value of the variation at Beverly in 
1781 was obtained from the mean of 7 partial means which did not 
differ more than 6 minutes from each other. The variation at Sa- 
lem in 1810 was the mean of 5125 observations. Confidence may 
accordingly be placed in both of these values; and consequently 
in the mean annual decrease of 1’.3 that results from them. Again, 
* The following values of the latitude and longitude are taken from the 
American Almanac for 1840. 
Latitude. Longitude. 
Boston 42°) 21! hives 
Cambridge 42 22 wh 
Salem 42 31 70 53 
