:

the plate AB, by means of the milled screw L and is fastened to the plate AB, by a clamping nut underneath the main plate.

172. To set off the variation of the needle on a vernier compass, turn the north end of the compass plate by the tangent screw L of the vernier (see Fig. 84), over the number of degrees in the variation, to the left (the observer is supposed to be standing at the south end of the plate and looking towards the north end), if the variation is westerly, to the right if it is easterly.

173. To run out lines of old deeds, set the compass upon one end of an "original line," determined to be such by old "marks," or by testimony as to its having been undisputed for twenty years or more, and turn the vernier plate till the reading is the same as that given in the deed; then run the other lines by the bearings and distances as given in the deed.

174. To make allowance for change in "variation in running out old lines, when only one "corner" can be identified, the surveyor must see to it that the date of the deed from which he is working is the same as the date of the survey from which the description of property in the deed is taken ; very often descriptions are copied from former deeds, in which case the date of the survey must be discovered before running the lines.

175. If no "original" line of the property can be found, nor a corner and date of survey, then the surveyor must seek his data upon neighboring property and work back from there, through several deeds if necessary.

176. It is very desirable that the date of the survey from

which a description is taken, and also the name of the surveyor, should be entered in every deed conveying lands.

The form of a "Survey Bill" may be somewhat as follows:

Description of Property in Town of

State of

-, County of

surveyed on the sixteenth day of August, in the year eighteen hundred and eighty-three, and bounded as follows:

Beginning at a point in the northwest corner of land owned by A. B. C., and marked by a cross cut into the top of a stone sunk three feet into the ground,-from which point the east chimney of the house of A. B. C. bore S 874° E, distant 8.56 ch.— thence from the point of beginning N 18° E, 11.27 ch. to a stake; thence N 56° E, 15.26 ch. to a corner where two stone walls meet; thence S 87° E, 5.56 ch. to the shore line of the bay; thence following the shore line to a point on the shore line bearing S 23° W, and distant in a straight line 13.47 ch. from which point the middle of the eastern key-stone of the bridge across Beaver Dam Creek bears S 37° E;-thence, following the middle of the road to a point in the middle of the road bearing S 87° W, distant in a straight line 5 ch.—thence, ....&c., &c.; the above described property containing, by cal culation, Acres, Roods, and

Variation of the needle

Perches, more or less.
ERASMUS RUGGLES,

BOOK IV.

TRANSIT

SURVEYING.

SECTION I.

SURVEYOR'S TRANSIT.

177. The transit is an instrument used for measuring horizontal and (when furnished with a vertical circle) vertical angles. It is placed on a tripod, TTT, to which it is screwed fast by means of a horizontal brass plate, DE. Upon the upper surface of the plate DE rest four screws with milled heads, called leveling screws, which work through the second horizontal plate, FG, into cylindrical nuts, shown in the figure. The two plates DE and FG are called leveling plates. The lower leveling plate, DE, is made in two pieces-the upper piece, which is screwed fast to the top of the tripod, having a large opening in its centre, in which the smaller lower one is shifted from side to side, or turned completely around.

66

By this arrangement, termed a shifting centre," the instrument is easily moved over the upper plate, and the plummet, which hangs from the centre, set precisely over a point without moving the tripod.

The upper side of the plate DE terminates in a curved surface, which encloses a ball that is nearly a hemisphere with the plane of its base horizontal. This ball or hemispherical nut is screwed fast to the smaller base of a solid conic spindle, that passes through the curved part of the plate DE. To this spindle is firmly attached the second horizontal plate, FG.

To this spindle, also, and above the plate FG, is fitted and fastened by spring-catch, a socket, called the main socket, to which is attached a third horizontal and circular plate, BB,

called the limb of the instrument. Fitted to the upper conical surface of the main socket is a second socket to which is united a thin circular plate, AA, called the Vernier Plate,

178. The sectional view, Fig. 86, shows the interior construction of the sockets of the transit, the manner in which it is detached from the spindle, and the means by which it can be taken apart if desired.

In the figure, the limb BB is attached to the main socket of LL, which is itself carefully fitted to the conical spindle P, and held in place by the spring catch C.

The upper plate, AA, carrying the compass-circle, standards, &c., is fastened to the flanges of the socket, UU, which is fitted. to the upper conical surface of the main socket L; the weight of all the parts being supported on the small bearings of the end of the socket, as shown, so as to turn with the least possible friction.

A small conical centre, in which from below is inserted the strong screw, S, is brought down firmly upon the upper end of the main socket LL, and thus holds the two plates of the instru