326 Application of Photography to the Self-registration 
nothing but extreme care, in cutting off every source of external 
or mechanical agitation, will give security to them Mr. Brooke 
has given a curious lithograph of a trace produced under extreme 
“local disturbance,” namely, a quadrille party in the house ad- 
joining, which is sufficient to shew that if the tremor communi- 
cated from such sources does not altogether annul the magnetical 
movements, it materially modifies them. It will also be found 
necessary to adopt some means of rapidly reducing the natural 
movements of the magnet, of which by far the best, is the applica- 
tion of a heavy copper ring or damper, as shown in the preceding 
diagram (f); if the bar * ce strongly magnetized, a fine wire 
dipping into mercury may be attached to it, ‘but where the mag- 
netism of the bar is feeble, or the horizontal component of the 
force has a low value, this method seems objectionable. 
We have already described the lamp. It will be seen that when 
placed before the mirror an image of the vertical slit through 
which the light passes will be formed in the conjugate focus: 
this image being condensed to a point, by the intervention of a 
lens of any kind, any angular movement of the magnet and mirror 
will cause a movement of that point through a space equal to 
twice the tangent of the angle, to radius the distance of the image 
from the mirror; the trace can therefore be made upon any requir- 
ed scale, by varying that distance. At 7 ft. 2 in. it will be 20’ to 
Linch, at 9 ft. 64 in. it will be 15’, and at 11 ft. 11} in. it will be 12’ 
tol ary the Panes. to be selected must be determined by the 
probable range of the element in ordinary magnetic disturbances, 
for since very great ranges are of rare occurrence and seldom 
more than momentary, it does not seem expedient to reduce the 
scale sufficient to include them, at the sacrifice of cnmenees in 
the more usual movements.* The scale at present in use at 
* The width of the half sheet of paper is sufficient for a range of 40/ of declina- 
tion and ‘008 of horizontal force, upon the largest scales likely, under any cireum- 
stances, to be adopted, namely 10’ ae declination and -002 of horizontal force, to one 
inch. It appears from observations at Toronto, from 1840 to 1849 inclusive, embrac- 
ing altogether about s, and including 1 ions of rvations, 
d that great ranges may be expected at that station in the llow 
Range of Declination. Range of Horizontal Force. 
Less than 40’ bk avis days.| Less than 008 . . 2974 days. 
From 0° 40’ to 0°50’ , From ‘008 to ‘010 oe hes 
0° 50’ to 1° 0 : : ‘OLO to OIE iy ven ke 
1°0’ to 1°10/ he ‘015 to 020 a ate 
1° 10’ to 1° 20/ q 020 to 025 9 
° 20’ to 1° 30 2 025 to 030 i 3 
1° 80’ to 1° 40/ 2 030 to 035 7 
1° 40’ to 1°50 1 035 to ‘040 3 
1° 50’ to 2° 0" 4 040 to 045 2 
F< ee 6 045 to ‘050 1 
8° st 4° 2 050 to -055 2 
ore than 4° “ ue More than ‘055 11 
On 211 of the term ‘i and bances, the of Seclinassoh fell within the 
ve s Walls (GP ead OT oo tases the ac ntal force fell within the 
