Lovering and Bond on Magnetic Observations at Cambridge. 61 
tion for any single moment were desired, we should readily find it 
by adding or subtracting as the case required the difference be- 
tween the reading on the scale and 100 to the absolute value of 
100. But when speaking of the absolute variation we generally 
intend some mean value which is the representative of that element 
for a whole day or month or perhaps a longer period. We should 
certainly miss of this mean variation if we adopted the regular maxi- 
mum or minimum reading of the scale or that of any extraordinary 
stride which may have been observed during the period. No single 
mark on the scale can lead to any thing more than a momentary ex- 
pression of this element. The mean of the daily maximum and min- 
imum limits of the scale or more accurately the mean of all the ob- 
servations furnishes a mark from which some durable value of abso- 
lute declination may be derived. Now the mean of all the obser- 
vations belonging to the 10 days of June is 103/603 of the scale: 
Hence the variation sought is . : F : : 9° 20! 59” — 3! 36” 
Saks oa lie (ee 
Again, the mean of the observations made during the 
5 days of October is 98/.838 ; hence the variation is 9° 17/ 29/4 1/ 10” 
or 9° 1s" 39!" 
The mean absolute Declination from June to Novem- 
ber, 1840, may be considered : : : = 9° 18 01”, 
The following Table shows the variation of the needle at Cambridge 
and in the vicinity from the period of the earliest observations: 
Cambridge 1708 go 0’ W. Cambridge 1788 6° 38’ W. 
es 1742 Same Ole se Boston 1793 6 30: «§ 
ee 1757 meOnnse Salem 1805 ST) tr 
es 1761 a ea pass Se 1808 5 20 « 
Ke 1763 6 AOE es 26 1810 (PI GG 
ce 1780 Tf ek gol Cambridge 1810 Taaoe 
Beverly 1781 [ist Ay oats $f 1835 SHS) ) 
Cambridge 1782 6 467 "5 es 1840 Gio dite 96 
re 1783 Gira 2i ac 
