Improvements in Circles for Surveying or Levelling Instruments.
Year: 1898

page_text: Date of Application, 30th Nov., 1898-Accepted, 29th Apr., 1899
COMPLETE SPECIFICATION.
Improvements in Circles for Surveying or Levelling Instruments.
I, JOHN COLEMAN FERGUSSON, Hastings Street, Vancouver, British Columbia,
Canada, Civil Engineer, do hereby declare the nature of this invention and in
what manner the same is to be performed, to be particularly described and ascer-
tained in and by the following statement:-
My invention relates to an improved attachment to be applied to terrestrial
measuring and levelling instruments; and the objects of my invention are,
first;
To provide a means whereby transits, theodolites, miners-dials, circumferenters
and all altitude and azimuth instruments when used with an ordinary engineers
levelling rod, will be converted into a self-indicating level and telemeter instru-
ment, giving the horizontal distance and the difference of levels between stations,
in feet, meters or any foreign measure that can be read at sight, without
requiring the use of any trigonemetrical tables or formula, for the reduction of
levels and distances, or for the correction of the rod for inclined sights; and,
second, to provide a means for dispensing with the formula
H =
2°
sin. 2, and D = cos.² a where is the stadium distance,
and all other equations necessary to correct the reading of the rod for inclined
sights, when stadia measurements are taken with a vertical circle, divided into
degrees, minutes and seconds; or when stadia measurements are taken with a
circle divided into four hundred grades, as in the centesimal division used with
tacheometers, and thus saving time in calculation; and the circle used as a
horizontal or azimuth circle gives latitude and departure, also length of the course.
I attain these objects by the mechanism illustrated in the accompanying
drawing, in which-
Fig. 1 shows my improved circle set vertically and attached to the transit or
theodolite.
Fig. 2 is an enlarged illustrated quadrant of the circle, scaled off tangentially,
and showing the method employed in reading levels and distances.
Fig. 3 shows my circle with one half divided off into octants, as indicated by
the letters i, and
Fig. 4 is an illustration of the method employed for obtaining the latitude
and departure, when my circle is employed on the azimuth plane.
Similar letters refer to similar parts throughout the views.
In Fig. 1, A indicates my improved vertical circle, which is substituted for
the ordinary vertical circle. This circle A is secured to the trunnion B resting
on the standard C of the transit or other instrument. Secured to the standard Č
are oppositely projecting vernier arms D) placed on a horizontal plane to the
instrument.
a and b indicate the centre of the verniers. These serve as indicators for
reading the tangent scale, while the verniers are used with the rest of the circle,
when readings are taken in degrees, minutes and seconds.
The octants in one half of the circle are each laid off tangentially into one

page_text: hundred primary divisions, in the manner illustrated in Fig. 2, from zero at c
in each case (see Figs. 1 and 3), towards the lines dividing the octants. The
other half of the circle is divided in the usual manner into degrees, so that vertical
angles can be read off if desired.
Fig. 2 shows the method of constructing the octants.
In illustrating the application of my improved circle the line de in Fig. 2 is
supposed to represent a radius of one hundred feet, the line f g represents a
quadrant of a circle, and the line eh represents a tangent to it, at the radius de.
The tangent is represented by a straight rod held vertically at the point e,
which is, as before mentioned, one hundred feet from the centre of the circle d.
This tangent, hereinafter called the tangent rod, is subdivided into feet, to
represent an ordinary levelling-rod.
A line drawn from the point d, to the point upon the tangent rod, ten feet
above the point e or to the Figure 10 on the rod, represents to the base de,
a slope of ten feet in a distance of a 100 feet; and a line drawn from the point d
to a point on the tangent-rod one hundred feet above the point e, represents to
the base a slope of one hundred feet vertical to one hundred feet horizontal,
or an angle of forty five degrees. The base being always an absolutely horizontai
line, which is obtained by levelling the telescope, a line drawn to any particular
division marked on the tangent rod represents the slope as a percentage of grade
which is read on the octant. An octant of the circle A is marked off with a
tangential scale on each side of zero at c; the measurement of falling grade is
performed in the same manner as for rising grades.
For example; the instrument and the telescope having been levelled, the arm b
on the vernier or indicator of the circle will always read zero, and the line of
sight through the telescope when the instrument and telescope are level, will be
the base line de; but by turning the telescope vertically, say, 5, 10 or 12
divisions upon the circle, the indicator of the instrument will mark a 5, 10 or
12 per cent. grade. And in like manner, by depressing the telescope vertically
the declivity is obtained.
In reading the horizontal distance of a station sighted to, the telescope is
turned upon its axis through one or more divisions of the circle, and the distance
passed over on the rod is noted: this distance covered on the rod multiplied by
one hundred and divided by the number of divisions, when more than one
division has been covered, gives the horizontal distance.
The term division is meant to express the figures 1 to 10, 20 and so on, on
the line eh, with their corresponding intersections of the distance on the
vertical circle A.
Although each octant of the circle is only divided into one hundred divisions,
each of which represent a foot, at one hundred feet, yet the ordinary levelling
rod is divided into fect, tenths of a foot and hundredths of a foot, therefore, whole
divisions upon the circle may be used for telemeter work, while tenths or
hundredths of a foot may be read on the rod at several hundred feet distance.
With a telescope of
22"
power, an ordinary "Sopwith" levelling rod can
be read on a clear day to one hundredth of a foot at a distance of six hundred
feet.
Fig. 3 shows the construction of the horizontal circle, half of which is laid
off in octants, which are read from two base lines, at right angles to each other;
one supposed to run north and south, and the other east and west. When the
course to be measured runs northerly or southerly then the base line running
north and south is used, and the deviation of the course from north is either cast
or west is measured on the adjacent octant, and the indicator records the per-
centage of the easterly or westerly deviation from north. Example; if the line
of sight pointed somewhat east of north and the indicator pointed to the division
north 15 east, then the course would be plotted by a line in the proportion of
15 feet east for every 100 feet north or 15% of easting from north along the whole
length of the course.

page_text: distances and angles may be read on the instrument with a minimum of calculation,
as herein before specified.
4. A circle to be attached to terrestrial measuring instruments, the combination
of one half of which is divided off in degrees, minutes and seconds, and the
other half divided into octants, and these octants being divided tangentially each
way from one or more base lines, which base lines are the divisions of the
octants.
Dated this 8th day of November A.D. 1898.
WILLIAM G. TRETHEWEY,
Agent.