MATHEMATICAL AND SURVEYING INSTRUMENTS. 59 
application of the operator’s foot. In surveying, two chain carriers carry 
the staves with the chain; the forward one has in addition a number of 
arrows or pickets (fig. 29). The chain staves are sighted in the line to be 
measured and driven in, the chain stretched tight between them. The 
chain is then carried forwards, the forward carrier taking with him the 
chain staff, inserting an arrow in the hole made by the latter. When the 
hind carrier comes to the arrow, he takes it up, and inserts his staff, over 
which the ring of the chain is slipped; the forward carrier then stretches 
the chain and inserts his staff, which he again replaces by an arrow, and 
the operation is continued until the line is measured, or the forward car- 
rier has exhausted all his arrows, which are generally ten in number. In 
this latter case a transfer of the arrows to the forward carrier is made, 
and the measuring proceeds as before. Careful count must of course be 
kept of the number of such transfers. This operation is sometimes car- 
ried on by means of the arrows and chain alone, the chain staves being 
omitted. 
Of the more complicated surveying or measuring instruments we shall 
first of all mention the plane table. This was invented by Pretorius, 
hence called mensula pretoriana, and is used to obtain a reduced plan of 
a tract of land. Of the various improvements made in this instrument 
since its invention, the two principal are represented in pl. 5, figs. 20 and 
21. Fig. 20 exhibits the instrument as modified by Major Lehman of 
Saxony. It consists of a stand with three feet, a, b, c, shod beneath with 
iron, and so attached to the stand by joints and winged screws as to allow 
a horizontal position of the table even on uneven ground. The support, J, 
upon which the board, m, rests, may be rotated on its axis; the board itself 
can be rendered perfectly horizontal by the three adjusting screws, g, h, 
and 7, passing through the plate K. At a later period Lehman did away 
with these screws, producing the adjustments entirely by the feet of the 
stand. He also changed the mode in which 7 was turned, by causing it 
to run out into a disk below, turning on the plate K. This disk had an 
endless screw turned upon its edge; a spindle attached to & caught in this, 
so as to produce a very slow rotation of the board. The spindle was 
capable of being removed for coarse adjustments. 
Fig. 21 represents a section of the upper part of Mayer’s plane table. 
It has also a three-footed stand, upon which the socket, a, fits. In the upper 
part of this socket is an excavation in which the ball or nut, 0, turns, capable 
of being fixed by the screw c. This nut carries above, the spike upon 
which the plate AB rests, and to which the metal ring, /k, is screwed. df is 
a portion of a middle piece with three curves, through whose extremities, f, 
pass three screws; one of them represented at A. These catch in the stir-» 
rups, gi, mn, upon which rests the ring /k, with its incumbent plate, AB. 
By these adjusting screws, the horizontal position of the table is secured. 
Instead of the female screw at g, a knuckle may be attached, as shown at 0, 
The screw e serves to fix the apparatus to the spindle. This form of the 
plane table does not give the fine adjustment of the first, as the screws allow 
a certain amount of spring. 
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