The Transactions of the Institute contain nothing, as yet, on the subject of pocket-compasses; and in the belief that American miners, explorers, geologists, and engineers will gladly welcome any information about new forms of apparatus, simple in construction and adapted to rapid use, I offer a short description of an improved French combination pocket-compass, which, if once introduced into use, would seem likely to supersede similar instruments of older pattern. The instrument in my possession I owe to the kindness of M. G. de La Bouglise, of Paris.
The frame of the compass is a double-bottomed box, 2½ inches square and 9/16 of an inch high, made of white metal, and with its surfaces very carefully planned. The sides of the box make true right-angles with each other. Two sides are parallel with, and two are normal to, the north and south line of the principal graduation. The bottom is accurately planed, so that the instrument will lie true on a plane surface. A glass, 2 3/8 inches in diameter, is set in the top plate.
The graduation for the needle is engraved on the upper face of a ring, connected with the flat disk, in which the center-pin is inserted. Between this disk and the bottom of the box there is a hollow space a quarter of an inch deep.
The needle is 1 7/8 inches in length, and can be balanced by means of a small sliding piece of metal. The graduation extends from 0° to 360°, reading from right to left. The stopping of the needle is effected by a screw, A, on the side of the box near the handle, H (Fig. 1). There is also a clinometer attachment, the graduation for which is engraved on the upper face of the silver-plated disk.
The compass has two sights. The north sight, B, is a simple pin, a little more than an inch high, which, when not in use, is kept folded down, and is partially protected by the exterior sliding-plate, C. When raised for use, the sight is stopped in a vertical position by a projecting pin attached to the side of the box. The south sight, D, is connected with the handle, H, and the sight and handle together are hinged to the extremity of a plate, E, an inch and a half long by half an inch wide, which can be slid through a slot in the side of the box, so as to rest on the bottom plate in the hollow space referred to above. From one side of the plate, E, a rectangular piece of metal has been cut for use in leveling, as will be explained further on. When the compass is not in use, the sight is folded down so as to lie flat on the sliding-plate, and the two are pushed into the box, so that nothing but the handle is visible on the outside. The sight itself, F, is a rectangular piece of glass, 1¼ inches high, so set in its frame that it has two reflecting surfaces, one on each face of the sight; the slit cut in the center lies in the vertical plane passing through the north and south line of the compass.
The distance between the two sights may be made longer than the side of the compass by sliding out, to its full length, the movable plate, E.
The handle, H, is flat and pear-shaped, and is movable about its hinge, so as to be capable of taking various positions, the extremes being when reversed and lying flat under the bottom of the box, and when standing perpendicular to the face of the compass.
The opening on the north side of the box, shown in Fig. 1, but not lettered, leads to a chamber in the hollow space beneath the disk. This chamber serves as a place of storage for a small ball- and-socket joint, one end of which can be screwed into a hole in the compass bottom directly under the center-pin, while the other end is provided with a screw by which the instrument can be attached to any convenient wooden support in either a horizontal or vertical position, according to the observation to be taken.
The compass is carried in a nice wooden case lined with velvet.
On the inside of the cover there is a mirror, in the middle of which a line has been scratched and colored red (Fig. 3). When the case is closed, this line covers the north and south line of the compass. The whole apparatus weighs eleven ounces.
When the compass is to be used for rough work only, it need not be taken out of its case ; the cover is simply opened, and the eye is so placed as to see on the red line of the mirror the reflection of the object sighted to, additional accuracy being secured by raising up the north sight, B. Closer results are obtained when the compass is taken from its case and placed on some flat surface, and both sights are used; but in order to secure the maximum of accuracy obtainable, the socket-joint must be used. By this means the compass can be fairly well leveled, and bearings can be taken with more precision. Any kind of a rest, even a walking-cane, can be used to screw the socket to.
When used as a leveling instrument, the sliding-plate, E, must be drawn out, and the south sight must lie flat upon it, the compass being held by the handle and allowed to hang freely from the hand (Fig. 4). In this position the sight will be in a vertical plane, and consequently the object seen through the vane, and the image of the eye in the mirror, will be in the same horizontal plane. Leveling can also be done with both faces of the sight, a small screw serving to adjust the sight in a true vertical plane.
The square form of the instrument facilitates its use as a clinometer, the dip or vertical angle being taken by resting the proper side on the surface or line whose inclination is to be measured. A vertical angle can be more accurately measured by using the socket- joint, screwed horizontally upon any appropriate stationary support, situated above or below the object sighted to.
On the back of the compass a table is engraved, giving the percentage of rise corresponding to given angles.
The instrument, as described, is made by M. Dutrou, 18 Rue Dauphine, Paris, and a patent has, I think, been applied for in the United States. I cannot give its exact cost, but I think it is about twenty dollars at the place of manufacture. In its present form it makes an excellent pocket-theodolite, deserving of broad acceptance, but a few additions or changes might, perhaps, be introduced for use in this country, such as:
- A lighter and more strongly magnetized needle, in order to lessen the momentum of the oscillations, and admit of shortening the time of observation.
- The introduction of a graduation showing bearings directly, to take the place of, or to be added to, the 360-degree graduation.
- By adding a hook to the north sight, and notching the handle, the instrument could be suspended from a line, and used in nearly
the same way as the German mining compass for taking horizontal or inclined courses in mines (Fig. 2).
The compass box being square, the plotting of observations on paper can be done at once, the lines being drawn with one side of the box as a rule, or, if the side is too short, by laying a longer rule against it. With the aid of this compass topographical data may be determined, if not with great accuracy, at least with considerable ease and rapidity. The ascertaining of the height of a summit and its horizontal distance from the place of observation, becomes a simple graphical operation after the necessary plotting of a previously measured base has been properly carried out.
A good, but not indispensable improvement would be to make the graduation circle movable around its axis, in order to set off the variation of the needle.