112 
the proper degree of the declination are correspond. The change of de- 
clination will cause a change of time on the dial of about eighteen 
seconds each year at noon. The declination at present is, I believe, 
25° 41’ west. ; 
The style is a human hair, which, passing up through a hole in the 
centre, whence the hour-lines are drawn, forms with the plate an angle 
equal to the latitude of the place, and is sustained at that elevation by 
a light arch of brass fixed to a thin brass bar which passes diametrically 
under the dial-plate, in the direction of its meridian line, to give stea- 
diness to the whole. Adequate tension of the hair, a hygrometric sub- 
stance, is secured by a small weight underneath, or by a weak spring. 
The perfect horizontality of the dial-plate is attained by bits of paper 
pasted to the under surface where required. The total weight ofall the 
parts just described should not exceed one ounce; so that the dial will 
be under the control of the magnetic bar, and will return, when dis- 
turbed, to its right position with the ereatest accuracy—a, condition 
without which the instrument were of little use. The steel pivot on 
which the dial turns is fixed to the dial-plate itself, and is supported on 
an agate hollowed just sufficiently to prevent its falling off. I find that 
the point of a fine sewing-needle answers best as a pivot, and with this 
advantage, that it may be so adapted as easily to permit replacement by 
a new one as often as required. 
To secure the dial against movements of the air, it is necessary that 
it be covered with a hemisphere of glass resting on the stand of the in- 
strument, in the manner of a French shade. The hemisphere, however 
necessary in this respect, produces an effect which, unless obviated, 
would render the dial useless: when placed in sunshine, a caustic of 
reflection illuminates the exact spot of the dial-plate where the shadow 
of the hair-style ought to fall; hence there could be no shadow. The 
obvious remedy is to paint the reflecting portion of the interior of the 
hemisphere with dull black paint. 
A lifter constantly sustains the weight of the dial, unless when it is 
in use, to prevent the effects of friction on the very sharp steel point. 
The whole instrument is mounted on a pillar, for redsons which will 
hereafter appear. 
When the radius of the dial is about five inches, the shadow of the 
hair-style, although weak, will reach the graduation in all parts, spread 
out by penumbra, it is true, towards its termination amongst the noon- 
tide long hour-lines; but still, if its centre be observed, adequate to in- 
dicate the time with sufficient distinctness for ordinary purposes. If 
greater accuracy be required, as for setting clocks and watches, where 
standard time-keepers are not accessible, the following mode of observa- 
tion will be found sufficient ; and herein is one of the peculiar advan- 
tages of this construction :—Let the observer place himself in such a 
situation that, one eye being closed, he can with the other see both the 
hair-style and the strongest and narrowest part of the shadow united 
into one line. At that moment, let him glance his eye along the hair- 
style to the graduation, where the hair will appear upon one or other of 
