size, and may be used upon a table of any form. It is convenient to suppose the north on the left hand, and the upper edge of the blade to represent the meridian of the station.

"This protractor is held in the hand while the vernier is set, which is an immense comfort to the sight; and it will be seen that, as both sides of the arm are parallel with the zero and centre, the angle may be drawn on the paper against either side, as the light or other circumstances may render desirable.

"From this description, and a mere glance at the plate, it is clear that angles taken with the theodolite can be transferred to the plot as accurately as the protractor can be set, namely, to a single minute; and that, too, in a rapid and pleasant manner.

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"By means of the notch at the end of the arm, this instrument may be used in the manner of a circular protractor, should a square not be at hand.

"This protractor is specially for plotting a survey, and therefore is figured from left to right; but should it be required for other purposes, to set off an angle from right to left, then mark off the supplement of that angle."

The experience of practical men is always highly valuable, and we are indebted to Mr. Howlett for the above useful remarks. His method of carrying the protractor forward is excellent, and the instrument itself is a very good one. Another protractor, however, on a similar principle, but possessing additional advantages, has been put into my hands by E. B. Metcalf, Esq., which I strongly recommend to the attention of surveyors, both military and civil.* (Plate XII.) The radius is 5 inches. Its arc is numbered from left to right 180°, and from right to left to 360°, by which the sup

* This gentleman was for a considerable time employed on the Trigonometrical Survey of England; and it was then that he contrived his valuable addition to the protractor.

plement of any arc to 360° is at once seen. Each degree is divided into three parts of 20', and the vernier reads to one minute. When set to 180°, it may be used with great exactness as a circular protractor, if so required. Thus far the instrument before us differs but slightly from Mr. Howlett's; but, by an ingenious contrivance, the radius is rendered the means of setting off distances at the same time that it protracts an angle: scales of 2 inches to a mile, both in feet and links, being marked upon the radius-having a double vernier, reading on one side to a single foot, and on the other to one link. Diagonal scales corresponding with these are marked on the ruler part of the protractor.

The manner of using the instrument is thus:-Let us suppose that the stations of a road-survey are to be laid down. Adjust the radius-vernier to the distance wanted, and turn the milled-head to keep it there: then set the vernier of the protractor to the required angle, and fix it by the milled-head, D: the instrument has only to be laid upon the paper, over which it is to be pushed by the T rule (according to Mr. Howlett's mode), until zero on the bevilled or fiducial edge of the radius agrees with the station-point. A common needle, set in a bit of wood for a handle, is now used to prick off the length of the station, and a line is drawn as far as the point marked. In this manner the distance is measured off with greater accuracy than could be done by the usual method of taking it with a pair of compasses from a scale of equal parts, while the process is also shortened ; besides, there is the further advantage of never having a greater length of line drawn on the paper than is actually wanted, and, consequently, the risk of adopting a wrong one is removed. For long stations, the centre of the protractor may be used instead of the zero given on the radius, the distances being taken along it from the centre. The object of having scales of feet and links, is to enable the

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surveyor to use the Gunter chain, or that of 100 feet, at pleasure, and he has only to note in his field-book whenever one chain is exchanged for the other, to avoid all chance of mistake. Much time may be saved by making use of the 100-feet chain on level ground; but when a surveyor is working with only one assistant, and, consequently, takes one end of the chain himself, the heavy chain becomes very fatiguing when surveying over a hilly country.

It is scarcely necessary to point out that a surveyor may have his scales on the radius to suit the nature of his work. A military man, for instance, might have one side divided to 4 inches, and the other to 3 inches, to a mile, which he would probably find more useful than having one side in links: then, by taking half the quantities, he might work to a scale of 2 inches, or 1 inch to a mile, if requisite. The numbers 50, 60, 70, &c., have reference to using the centre of the protractor instead of the zero, when long distances are required.

Notwithstanding the superiority of moving the protractor forward by means of a T ruler, I think military men will often be disposed to use the large three-bar parallel ruler on account of its portability. One bar is fixed, either by means of leaden weights or small screws, while the others advance over the paper, pushing forward the protractor.

There is another method, which ought to find a place here, being still more suited to our wish, for portability, as it requires no ruler to work the instrument with. A number of parallel lines are ruled upon the paper at about an inch or so apart, to represent meridians, which are marked north and south. The semicircular protractor is then set to the required angle: a point is next marked on the paper for the first station, and the protractor is then adjusted to any convenient meridian so that the ruler part or base of the instrument may coincide with one of the meridian lines,