A Shadow Compass or Instrument for Taking Bearings (Nonmagnetic).
Year: 1893

page_text: Date of Application, 2nd Dec., 1893-Accepted, 10th Feb., 1894
[Communicated from abroad by STEVEN WATKINS, of Smyrna, Turkey, Civil
Engineer.]
A Shadow Compass or Instrument for Taking Bearings
(Nonmagnetic).
I WASHINGTON HUDSON of 2 Elliscombe Villas Charlton in the County of
Kent Consulting Engineer do hereby declare the nature of this invention and in
what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement:—
The object of my invention is to determine by a nonmagnetic compass the north
or meridian and to take bearings by means of the suns shadow on a time instrument
specially constructed and devised see accompanying drawing in which the same
letters represent like parts in each figure.
Fig. 1, is the revolving dial or watch face with two pins A, A, and wire C, also
an arrow line marked on face immediately under and in a line with wire.
Fig. 2, is the annular fixed dial with figures engraved or marked so as to appear
in their proper position when viewed (inverted by reflection) through the prism.
Fig. 3 is a section of the instrument in which A, A, are the upright pins
supporting the wire C. B is a revolving dial. D is the annular fixed dial. E, the
space for clockwork. M, the glass cover. N, the hole for winding.
Figs. 4 and 5 are two views of the instrument in which F is the radial arm
turning on a centre, and hinged at G for folding when closed, as shewn by the
dotted lines. His a plate carrying a prism, J, and having a slit K all sliding in
sockets at O, O. L is a vane pin or pointer.
The shadow compass or instrument for taking bearings by means of the suns
shadow consists of a round metallic box, in the middle of which is fixed a handless
watch wound up in the usual way and constructed in such a manner that the face
or dial revolves and makes one revolution in every twenty four hours. A thin line
representing an arrow is drawn on the dial through the centre. At the extremities
of this line two uprights or pins of about one quarter of an inch in height are fixed,
and a fine wire or thread is stretched across from one to the other, so as to be
perfectly parallel to the dial, and in the same vertical plane as the arrow line.
In the top of the box is provided a rim or annular dial the inner diameter of
which is made somewhat larger than that of the watch face or dial, so as to allow
the latter to revolve freely within, both rim and dial being flush that is brought to
the same horizontal plane.
The outer periphery of the annular disc is graduated into 360 degrees and half
degrees and the inner periphery or hour circle divided into 24 hour parts each part
denoting 15 degrees and figured as shewn that is XII, or noon to be inmediately
under 360 degrees VI, A.M., to the left opposite to 270 degrees and VI, P.M., to the
right opposite 90 degrees of the circle.
A radial bar or arm is centred on the underside of the box and moves round it
carrying with it a prism fixed in a sliding plate having a slit through which the
vision is directed when making an observation.
The plate carrying the prism can be raised and lowered at pleasure either by
gear or friction, and clamped in position.
The radial arm is hinged or jointed so as to open into the perpendicular position
against a stop when in use and folded down when not in use, or it may be unshipped,
without the hinged joint as most convenient.

page_text: The sight vane or pointer is to cut the point to be observed when seen through
the slit and is fixed close to the cover or glass so as to avoid any shadow falling on
the top of the box except that from the wire on the dial.
One of the faces of the prism is made slightly convex, and of such a focal length
that the distinctest vision is obtained at the same time of both the greater portion
of the graduated circle and of the wire stretched across the dial.
When the watch is wound up and set to the correct time, the sun and the watch
turn at the same rate of 15 degrees per hour, but in different inclined planes, except
at the equator, where the shadow is always perpendicular in other latitudes the
arrow of the watch would not be always pointing to the sun as the latter travels in
its diurnal path, consequently the instrument must be turned round a little until the
shadow projected by the wire is in exact coincidence with the arrow line and when
in this position shows the azimuth or bearing of the sun and the north and can be
read off the graduated circle by looking through the prism.
The method of using the instrument for the purpose of taking the bearing of an
object is as follows-Steadily hold the instrument as nearly level or in a horizontal
plane as you can judge, and point the arrow to the sun but in such a position that
the shadow from the wire exactly covers the line on the dial. Raise or lower the
prism in its sockets till both the divisions on the graduated circle and also the
arrow are distinctly seen through it. Turn the radial arm round until the object
to be observed is seen through the slit in exact coincidence with the pin, still
keeping the shadow perfectly over the arrow line, then reading the divisions upon
the graduated circle which gives the azimuth or bearing of the object or the angle
which a straight line drawn from the eye to the object makes with the true
meridian.
.
The chief merits of this instrument are that the shadow cast on the dial is much
steadier than the magnetic needle in ordinary compasses which is often very
seriously effected by attraction.
Bearings can be taken in hurried surveys on horse back without the necessity of
getting down each time an observation is required, and may prove of great use for
military purposes, owing to the rapidity with which bearings can be taken.
In countries where the sun shines for the greater part of the year, it will be
found most serviceable when used on a tripod stand for filling in the details of
extensive surveys.
When used as a portable instrument any number of bearings can be taken round
the observer, excepting about 60 degrees on which the shadow of the observer
would fall thus obscuring that of the wire.
I sometimes fit a magnetic compass in the bottom of the case (in which the
instrument is kept), for use in cloudy weather.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed I declare that what I
claim is
An instrument for taking compass bearings and angles from the meridian by the
suns shadow in combination with a time dial and prism constructed and combined
substantially as and for the purpose described.
Dated this 2nd day of December 1893.
WASHINGTON HUDSON.