AN IMPROVED INSTRUMENT FOR MEASURING DISTANCES
Year: 1883

page_text: PROVISIONAL SPECIFICATION left by Alexander Melville Clark at the
Office of the Commissioners of Patents on the 8th November 1883. Com-
municated to him from abroad by the firm of. Dany and Lepage of Paris,
France
ALEXANDER MELVILLE CLARK of the firm of A. M and W. Clark of 53 Chancery
Lane in the County of Middlesex, Fellow of the Institute of Patent Agents AN
IMPROVED INSTRUMENT FOR MEASURING DISTANCES [communicated to him from
abroad by the firm of Dany and Lepage of Paris, France
This Invention relates to an improved instrument for measuring distances
whereby the indication of the distance of the object observed from the station is
obtained immediately by a single motion of the alidades without measuring a base
line or the angles made by the relative displacement of the sights or telescopes.
In principle the instrument consists of a frame mounted on a ball and socket
joint so as to be capable of motion about this joint in different planes and carrying
at the lower part a fixed telescope or pair of sights and a second telescope or pair
of sights mounted above the first and pivoted so as to be movable in the vertical
plane. The greater or less angles made by these two tolescopes or pairs of sights
correspond to the different distances of the station from the object sighted and by
this greater or less opening of the angle made by the sights, telescopes or alidades
a needle is caused to traverse over a graduated limb. As the position of the
needle corresponds to greater or less angles which themselves correspond to different
distances it will be understood that the scale having been graduated once for all by
actual experiment, all that is necessary after having sighted the object first
through the fixed telescope or sights and then without changing the position of
the instrument through the movable one is to read off the figure indicated by the
needle on the graduated limb in order to know exactly the distance it is desired to
measure.
In order that the Invention may be more readily understood I have illustrated
examples of the instrument in the annexed drawings and will proceed to describe
it in detail with reference thereto.
Figure 1 represents in an elementary way a front elevation of the instrument.
Figure
a plan of the base of the frame. Figure 3 is a face view of the upright
to which the movable telescope is pivoted by one end and Figure 4 is a face viçw

page_text: of the other upright showing the adjustment for the free ends of the movable
telescope. a is a bar mounted on a ball and socket joint on a foot b. c and g aro
two uprights rising from the ends of the bar a. d is the fixed sight tube or
telescope secured to the uprights c and g and is tho movable sight tube or
telescope pivoted at one end on centres e in the upright c and supported at its
other and movable end by a seat k on the point of an adjusting screw i screwed
through a nut on the upright g by means of a milled head j. is a friction
pinion on an axis mounted in bearings in the frame g and rotated by frictional
contact with a sector o fixed on the movable telescope or sight tubo fand m is a
needlo fixed on the axis of said pinion and moving over a graduated limb n A
vernier may be applied to the graduated limb if necessary for reading fractional
divisions.
I do not however limit myself to the above described elementary construction
and arrangement of the instrument, but amongst other possible modifications it
may be constructed as represented in Figures 5 and 6 whereby the indications are
rendered more sensible and the instrument is mado susceptible of more general
application to the different geodesical operations to which it may be necessary to
have recourse. As will be seen by these Figures the instrument is mounted on a
geodesical circle a and the circular graduated limb b is therefore horizontal when
in its normal position, and the needle is replaced by an alidade c whose extremity
is provided with an index which corresponds with the divisions of the graduated
limb b and which is moved automatically by a maltese cross wheel & through the
medium of a small rod e; the wheel & has as many notches as the limb b has circles
of division.
The instrument is provided with two telescopes, ono ffixed and the other g
movable; the latter is adjusted by turning a spindle h geared by pinions & with a
tangent screw i engaging with a worm wheel Zon whose axis is fixed a pinion m which
gears with a toothed sector n pivoted at o whose position is capable of adjustment.
"The heel p of the sector n is toothed and gears with a rack carrying an index
which shows on a scale r the number of turns of the alidade adjustment. This
mechanism is mounted on a ring or plate a mounted between the geodesical circle a
and the graduated limb b.
To obtain a quick return motion of the alidado to its initial position the tangent
screw is mounted on a support which allows of the screw being engaged or
disengaged at will from the wheel by turning the screw u.
The teeth of the different pinions and racks of the instrument are of little depth,
being in fact little more than milling, and to ensure their constant engagement
and adjust for wear, it is only necessary to turn one or other of the tightening
scrows v.
A plumb line is attached at any convenient point beneath the supporting plate
of the instrument.
The principle and operation of the instrument remain the same as before and
with the instrument thus constructed I may combine a vertical sector or limb for
measuring altitudes.