horizon glasses, fixed at an angle of 45°; hence, while viewing an object by direct vision, any other, forming a right angle with it at the place of the observer, will be referred by reflection, so as to coincide with the object viewed. Thus a line may be laid out perpendicular to a station-line, and from any point on it, by simply standing with the instrument over the given point, and looking through it along the line, having a person to go with a mark or station-staff in the direction the perpendicular is required, and signing to him by hand to move to the right or the left, until his staff is seen by reflection to coincide with some object on the line along which the observer is looking, when the place of the staff will be in a perpendicular to the station-line at the place of the observer.

If it be required to find on a line the place of a perpendicular from a fixed object, as a house, &c., the observer himself must move along the line until the image of the object appears, as before, in the direction of the line, and the place where he then stands will be the spot where such perpendicular would fall. This instrument has the advantage of great portability, not being larger than a shallow circular snuffbox, which it resembles in shape.

I believe we are indebted to Frederick W. Simms, Esq., C.E., for this highly useful little instrument, which I strongly recommend to all engineers and surveyors, whether civil or military.

DESCRIPTION AND USE OF THE PLANE-TABLE.

Although the plane-table has been in a great measure laid aside by English surveyors, both civil and military, some description of it, with an explanation of the manner of using it, ought to find a place in a manual of military surveying, especially as it is still in use at the Royal Military College.

Before the theodolite came into general use, the planetable was a capital instrument among surveyors, and continued to be employed, both for filling in and making small surveys, long after the introduction of the theodolite. For military purposes, the chief objection to the plane-table is the inconvenience of carrying it, even under its most portable form, as compared with the prismatic compass and sketching-case; but when surveys are not required to be performed with very great accuracy-such as the class of military sketches-they can be executed with great ease and rapidity by the instrument in question.

The old plane-table was an unwieldy affair, of some 15 or 16 inches square, which has very properly been discarded; and those surveyors who still adhere to this instrument, commonly use a board of from 10 to 12 inches square; they also reject the brass ruler furnished with sights, being contented with a common flat ruler.

In the adjoining figure, representing a plane-table, C is a small compass attached to the board; Dis a ruler detached, and lying flat upon it; R is a rim or frame, which serves to confine the paper to the board. The instrument stands upon three legs.

C

D

R

" In preparing the plane-table for use" (says Mr. F. W. Simms), "the first thing to be done is to cover it with drawing-paper; the usual method of doing which is the same as that of covering a common drawing-board, by damping the under side of the paper, and laying it on the board in an expanded state; press the frame into its place, so that the paper may be squeezed in between the frame and the edge of the table; and the paper, shrinking as it dries, assumes a flat surface for the work to be performed upon. There is one great objection, however, to this mode of putting on the paper, as, when it has once been damped and strained, it is easily acted upon by any change in the hygrometrical state of the atmosphere. We therefore prefer putting the paper on dry, taking care to keep it straight and smooth whilst pressing the frame into its place; but it must be acknowledged that this cannot be done so nicely as when it is damped. We have been informed that, if the under side of the paper be covered with the white of an egg well beat up, it may be laid on the board with the greatest nicety; and that when so prepared it is not easily affected by atmospheric changes."

AO

B

To show the manner of using the plane-table, we will suppose that a military sketch of a portion of country is to be made, and that the first proceeding is to determine the situations of certain points, as A, B, C, D, and E. This is effected by measuring angles from each end of a base line, and determining the positions of the points by intersections, the same as if working with a theodolite or compass.

E

D

Let the line, A E, be considered as the base, whose length is to be ascertained by the chain or pacing. Set up the instrument at E, making it as level as possible, by shifting the legs, judging by the position of the compass-card when the board is nearly horizontal. Draw a line upon the paper, and lay off the distance from E to A; then insert a stout needle, fixed in a piece of wood, through the point, E, into the table; and having laid an edge of your rule along the line EA, turn the table in its socket, until the distant point, A, is found to be in line with the edge of the ruler; in which position the table must be firmly clamped, by means of a screw for that purpose. Observe the reading of the compass while the table is in that position, and you will afterwards, wherever you may be, always have it in your power to set the table parallel to its first position, by giving the compass-card the same reading, and also be able to check your future operations. Keeping the edge of the ruler touching the needle, move it until it is in line with B, with C and D, drawing lines in succession in the direction of those objects.

This effected, the table is removed to A; and, in setting it up, be careful that the point representing A in the paper be exactly over the station-point and not the centre of the board. Fix the needle in the point, A, and having laid the ruler again along the line, A E, turn the table until its edge is found to be in line with the distant point, E; when the reading of the compass will be found the same as at the former station. Lines are then drawn in the directions of B, C, and D, which, intersecting those drawn from E, will give their respective situations on the plan.

If the plane-table is to be used for filling in between points previously fixed, some of the points are first transferred to a paper on the table; the instrument is then set up at one of those points, and, being turned round until a line joining that with another of the points is seen along the edge of the ruler to cut the distant object, the figure or lines transferred to the paper on the board are then known to correspond with those on the ground. The reading of the compasscard is then noted, and afterwards, at any point where the table may be set up in the course of operations, it will always be parallel to its first position, when the compasscard has the same reading.

To determine any particular place, such as the bend of a road or river, from whence two or more points already fixed can be seen, it is only necessary to set up the table over the point to be found, and turn it till the compass has the same bearing as at any one of the stations; when the sketch on the table will be parallel, or correspond with the ground, if there is no local attraction to interfere with the needle. Clamp the table, and fix a needle in the point representing one of the stations; place the ruler in contact with this needle, and then turn the ruler until the station is seen along its edge, when a line is to be drawn on the paper. The needle is then removed to another station on the board, and, the same proceeding being gone through, the intersection of the two lines drawn on the board will give the required point. But, as a proof of its correctness, a third line from another station should, if possible, be obtained, which ought to pass through the same point.

It is perhaps unnecessary to dwell longer upon the planetable, as the few hints above given will put any one in the way of using it, should he have occasion to do so; but I cannot omit some useful observations, with which I have been furnished by a very able surveyor, E. B. Metcalf, Esq., Professor of Military Drawing at Addiscombe College; who states that, "to be convenient for military purposes, the table should be small, not exceeding eleven or twelve inches square; having a frame (which may be divided into inches and tenths, to serve as a scale), to lift off to enable the tracing or paper to be laid on the board; which frame may be confined by means of buttons, or a piece of fine whipcord. The table should be of pear or lime tree, as those woods do not easily warp, and are besides soft, so as to enable the needle to be inserted easily; the needle must be fixed into a piece of wood for convenience, and the ruler ought to be divided for laying off distances, and rendering a pair of compasses