fo, or to of an inch. This is the least space that can be measured by means of the vernier, and is called the least count; hence,

The least count of a vernier is equal to one of the equal divisions of the limb divided by the number of spaces on the vernier.

169. The true reading of an instrument, for any position of the vernier, expresses the distance from the point where the graduation on the limb begins, marked 0, to the 0 point of the vernier. In the diagram, that distance is expressed by nine units of the limb, or 9.

If, now, the vernier be moved till the division 1 coincides with the division 10 of the limb, the 0 point will have advanced along the limb a distance equal to of one space, and, if we call each space b, the reading will become 9+b. If we again move the vernier till the division 2 coincides with the division 11 of the limb, the 0 point will have advanced an additional distance, equal to b, and the reading becomes 9+b; when 3 coincides with division 12, the reading will become 9+b, and so on, till finally, when the point 10 coincides with 19 of the limb, the distance 9 will have been increased by 8b, and will become 10, as it should, since, in that case, the 0 point will have been moved a whole space, and will coincide with the division 10 of the limb. Hence, the following rule for reading an instrument which has a vernier :

Read the scale, in the direction of the graduation, up to the line preceding the 0 of the vernier; this gives the number of whole units of the scale. Look along the vernier and find which of its lines coincides with a line of the scale; this line of the vernier gives the number of fractional parts of one unit of the scale to be added to the former reading.

170. In order to read a vernier correctly, especially if it be one with which we are not familiar, it is necessary to estimate, by the eye alone, the fractional part of one unit to be added; then if the vernier reading is nearly the same as the estimate, we may record the reading with confidence; but if the estimate and the vernier reading disagree largely, then the cause of such disagreement will probably be a false reading of the vernier.

171. In a vernier compass, the vernier is attached, as shown in the figure. A small arc HI is described on the bar AB, having its centre at the centre of the compass-box. This arc is divided to degrees, and sometimes to the parts of a degree. The vernier is permanently attached to the compass-box.

When the 0 point of this vernier coincides with the 0 point of the graduated arc HI, the north and south line of the compass-box lies in the plane of the sights.

The compass-box is turned about its centre, without moving

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:

State of

Description of Property in Town 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 north west 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 871° 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 calculation, Acres, Roods, and Perches, more or less.

Variation of the needle

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.

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.