uy 114 
exact coincidence is insured by bisection of the shadow of the black dot ; 
and if that shadow be made to fall very near the graduation, the hour 
will be precisely and distinctly indicated. The whole operation, which 
has taken so long to describe, may be performed in three or four seconds. 
I found that a large double convex lens also answered for this pur- 
pose admirably, with the exception that, as it required to be held at a 
great distance from the dial, 1t was very inconvenient. 
As movable sundials are intended chiefly for occasional service, and 
in cities and towns are best adapted for use in the house, attention is 
required in the selection of a proper situation. The magnetic bar, it 1s 
true, will determine the meridional position of the dial; but that posi- 
tion is liable to be disturbed by the numerous appendages belonging to 
a house, such as massive fire-grates, window-bars, balconies, locks, 
hinges, and even nails. The dial is naturally brought to the source of 
light, the open window; and indeed there is seldom any other situation 
where it can conveniently be exposed to full sunshine. There, how- 
ever, independently of the before-mentioned causes of derangement, an 
unsuspected one lurks unseen, which exerts a more detrimental influence 
than all the others together. On each side of the window are the sash- 
weights, which, beside their passive reaction on the magnetic bar of the 
dial, are generally magnetic in their own nature. When the lower sash 
is raised, its two weights are lowered, and brought within the influence 
of the magnetic bar; and, in consequence of their inequality of attrac- 
tive power, it is difficult to find an intermediate spot where they neu- 
tralize each other. Besides, the sun may not be then shining in the in- 
termediate spot. For this, and other reasons assigned, it is necessary 
that the dial should be furnished with means of obviating the disturbing 
influences which may cause the instrument to show erroneous time, as 
I have found it to do even to the amount of half an hour, under some 
circumstances. The remedy is to be found in the following appendages:— 
Erected on the stand of the dial is a brass pin, tapered off to a fine 
point, which, being bent at right angles over the declination arc, and 
very close to it, indicates how many degrees (if any) the dial-plate is out 
of its proper position for showing the hour. This pin may be distin- 
guished by the name of the Indicator. 
A brass bar, fixed to the stand of the dial, and projecting beyond its 
circumference, supports a very small ball of soft iron by an arrangement 
which permits it to be approached towards the magnetic bar on either 
side, or removed from it, as the case may require; or it may be turned 
away entirely. 
An arm of wood, consisting of three thin narrow slips, so jointed 
that they can be folded in when not in use, or drawn out and tightened, 
is movable round the centre underneath the stand. When drawn out, 
the position of the bar is horizontal ; at its end is a light card-paper box, 
in which a graduated circle is drawn. This circle is traversed by acom- 
pass-needle sustained on a steel point. By the length ofthe arm, which 
is two feet, the compass-needle, a very small and light one, is removed 
out of the influence of the magnetic bar, as well as of the mass of iron 
from which disturbance was apprehended. 
