A. D. Bache on Declinometer Observations. 199 
ter and summer variations, the oii being the difference be- 
tween those of the yearly curve, and of the winter and summer 
curves respectively. This di ingrem shows very perepicnoualg 16 
progress of the annual variation at different hours of t e day. 
It shows that at 6 or 7 A. M. the annual variation isa maxi- 
_ disappearing at a quarter before 10 A.M. That it reaches 
second, Aer apis maximum at 1 P. M. nearly disappearing 
shor tly after 5 p. A still smaller maximum is reache 
. M. and half oe hour before midnight the annual variation 
again disappears. At, and: before, and after the principal maxi- 
um between 6 and 7 A. M. the annual variation causes the 
‘ioctl end of the magnet to be deflected to the eastward in the 
summer and to the westward in the winter. At1 Pp. M. the deflee- 
tions are to the west in summer and to the east in gene the 
range of diurnal motion being thus increased in the former sea- 
son and diminished in the latter. The needle is — — 
in summer, more to the east in the morning hours more 
the west in the afternoon hours, or has greater elongation Stas 
it would have if the sun moved in the equator. In winter _ 
reverse is the case. The range of annual variation from sum 
ii to winter at Philadelphia is about 3’, and its daily range 
about 2’°6 
In diagram C similar curves are given, = ag PRES hs it 
cae and Hobarton, of the winter and su 
gress of the annual variation. The comparison aie that 
Toronto and Hobarton are not normal types of the half yearly 
deflections and the near coincidence of the forms at Philadelphia 
and St. Helena, seems to show that the type for different places 
is one and the same in general character, affected by incidental 
irregularities. 
In reference to the annual variation, Gen. Sabine, in the rectifi- 
cations and additions to the last volume of Humboldt’s Cosmos, 
presses himself as follows 
“Thus in each nelgpliehs the semi-annual deflections concur 
nsequently ——— them, and oppose and diminish them in 
ihe other half. At the magnetic ees as : no mean die 
m 
noon when the sun has moat declination and the reverse when 
south declination. » to the same authority the annual 
variation is the same in ie ake bamepieaes the north end of the 
magnet being deflected to the east in the forenoon, the sun hav- 
ing north declination, whereas in the diurnal variation the north 
