IMPROVEMENTS IN CONSTRUCTING SURVEYING AND OTHER INSTRUMENTS IN: WHICH THE MAGNETIC NEEDLE IS USED TO TAKE BEARINGS OR COURSES
Year: 1870

page_text: PROVISIONAL SPECIFICATION left by James Naysmith at the Office
of the Commissioners of Patents, with his Petition, on the 26th
July 1870.
I, JAMES NAYSMITH, of Eaton Town, near Swansea, in the County
of Glamorgan, do hereby declare the nature of the said Invention
for "IMPROVEMENTS IN CONSTRUCTING SURVEYING AND OTHER INSTRUMENTS IN:
WHICH THE MAGNETIC NEEDLE IS USED TO TAKE BEARINGS OR COURSES," to be as
follows:-
This Invention has for its object improvements in constructing
surveying and other instruments in which the magnetic needle is used
to take bearings or courses.
The circumferentor or magnetic instrument used in surveying consists
of a case with slight points upon it containing a magnetic needle
suitably suspended and a graduated dial which can be accurately set to
the needle and then read by means of a vernier.
According to my Invention I apply to the dial an inner circle divided
into degrees the same as the outer one, this inner circle is able to revolve
on the centre on which the needle is fixed, and can be fixed to the outer

page_text: circle at the desired variation by the two set or clamp screws. The inner
circle can be turned to the variation of the needle by a small pinion
fixed in the outer circle and working in suitable rackwork on the under
part of the inner circle. The said pinion is so constructed and fixed as
not to interfere with the working of the rackwork and pinion of the
outer circle, as at present used in taking angles. The pinion and the
set or clamp screws are turned by a small key similar to a watch key
and only a little stronger than the ordinary watch key. The set screws
and clamps fixed to the inner circle are able to traverse along two suitable
grooves in the inner circle, and the said grooves are of sufficient length
to allow the inner circle to turn at least 45 degrees each way, that is, the
north end of the inner circle to move 45 degrees east or west of the
magnetic meridian. The rackwork on the inner circle need only be
of sufficient length to allow the pinion to turn the same 45 degrees
each way so as to correspond with the grooves of the clamp screws.
The improvement as applied to the mariners' compass consists of an
inner circle marked with degrees being fixed to the magnetic needle, and
the outer circle (to be adjusted to the true meridian) to be marked with
degrees on its inner edge to correspond with the fixed part, the said
outer circle being capable of being moved around the inner circle in a
groove so that it may be turned to any desired variation. The outer
circle may be turned by hand, and it is fitted to such nicety in the inner
groove that it will remain stationary after being moved, or if desirable
it may be fixed to the inner circle by small clamp screws.
It will be observed that in the first part of the Invention as applicable
to surveying instruments the inner circle next the needle will indicate
the true meridinal bearings, while in the second, as applied to the
mariners' compass, the outer circle gives the true meridinal bearings or
courses.