the observer, and that the instrument may be above the height of brushwood, &c., which often impedes the view.

I shall not here enter into a description of this wellknown little instrument, which, however, will be found further on a quarter of an hour at any optician's will suffice to make any one acquainted with its use.

N246 B

70°

E

C

In surveying with the compass, the bearings of objects are taken from the magnetic meridian. Let NS represent the magnetic needle or meridian, W the west, and E the east; and suppose the sights of the compass are directed to the spire A; then if the angle NCA be 70°, for example, the spire is said to bear 70° w north-east, or 70° from the magnetic north towards the east.

The variation of the needle is its deviation from the true north. In this figure, the angle NCB represents the variation, B C being considered as the true meridian. The variation is now about 24° west, at London.

Having made himself master of the instrument, so as to be able to take bearings, I recommend the student to attempt the sketch of a road; the mode of doing which I shall endeavour to render very easy to him. He will require to be provided with a sketching case, about ten or eleven inches square, an ivory rectangular protractor, such as every case of mathematical instruments is furnished with, and a black-lead pencil. His paper, asses-skin, (or what is preferable to either, a piece of paste-board,) fits into the sketching case, and must be ruled all over with very fine lines, exactly parallel to each other, and at unequal distances, varying from a quarter to three-eighths of an

inch apart.* The purpose to be answered by ruling the lines at unequal distances, we shall presently see. The protractor is six inches long, and an inch and threequarters wide, having across it a number of very fine lines at right angles with its length, and at equal distances from each other,† constituting a scale of equal parts. For instance: if the distance between two red lines be taken as 100 yards, and the smallest divisions as 10, then the scale is that of four inches to a mile-seventeen large and six small divisions measuring exactly four inches. Hence, if a scale of eight inches to a mile be required, the distance between two red lines is 50 yards, and the small divisions become 5 yards each; and if a scale of two inches to a mile be required, the distance between the red lines becomes 200 yards, and the small divisions 20. But we may call the divisions on the scale 10, 20, 30, or any number we please, of yards, feet, links, or paces.

Sketching blocks have lately been used for sketching and plotting in the field, and found very convenient. Each sheet of paper composing the block has parallel lines printed on it from a steel plate, which not only ensures the accuracy of their parallelism, but gives the lines very fine, which is a desideratum. A useful size for a block is twelve by ten inches, and it fits into a patent leather case, which may be slung over the shoulder or attached to a saddle. They are made by Messrs. Reeves, 150, Cheapside.

It should be mentioned that these lines will not be found upon all ivory protractors; but those supplied by Messrs. Troughton and Simms, and Mr. Elliot, of Holborn, generally contain them.

SECTION II.

METHOD OF SKETCHING A ROAD BY MEANS OF THE COMPASS, AND PROTRACTING IN THE FIELD.

THE compass is set up at the point from which we purpose to start, as A (plate II., fig. 1); and care being taken that the card be level, so as to play freely, a mark is placed at B, where the road turns to the right; the sights of the instrument are then directed to the mark at B, and we find, when the card has settled, that the vertical hair cuts the 29th degree; the bearing of B is therefore 29° northeast-the mark at B lying towards the east point from the magnetic north.-I would observe, that in ordinary road sketching, it is not usual to set up any marks: some object will present itself, which must be kept strictly in line when pacing the distance; thus, much time is saved.

Next, to protract, or lay down the bearing of B upon our paper, which has upon it the parallel lines at unequal distances, to be considered as east and west, and therefore not meridian lines, which are north and south.

The protractor has across it, as we have already noticed, a number of lines. Now, fix on any convenient part of the paper, make a dot, and surround it with a small circle, thus, which denotes a station. Place the centre of the protractor at this point, and endeavour to make any one of the lines which are drawn across it, to coincide with one of those upon the paper, being careful to keep its centre very exactly on the point. This being effected, the protractor is