of any error that may exist in the adjustment of the vernier or zero of altitude.

It is of great importance that the telescope and vertical arc should move in the same vertical plane: in small theodolites, this is provided for by the construction, and no means are afforded for making any alteration; but larger instruments are furnished with screws for this adjustment. To prove the accuracy of the vertical arc, suspend a weight by a long plumb line from a branch of a tree, or otherwise, and adjust the cross hairs upon it; then, by means of the elevating screw, cause the vertical arc to rise, and observe whether the cross hairs continue on the line while the vertical motion continues. Or fix them on the angle of a lofty building, and raise or depress the telescope, when the cross hairs will continue to move on the angular line if the adjustment be perfect. For depression, a reflection of the same angle of a building from an artificial horizon will serve

as a test.

For a fuller description of the theodolite, if necessary, as also of other instruments, I recommend Mr. F. W. Simms' Treatise on the principal Mathematical Instruments employed in Surveying, Levelling, Astronomy, &c.; a work likely to be very useful, as the author combines, with a perfect knowledge of the subject he treats, a style of expressing himself, at once clear and concise. The same writer has likewise published a valuable little book on levelling.

DESCRIPTION OF THE PRISMATIC COMPASS. [PLAte 1.]

The box contains a card under which, and attached to it, is a magnetic needle; the whole nicely suspended on an agate point, so as to allow of the card playing freely. The circumference of the card should be divided to one third of a degree, or 20 minutes; but a bearing may be estimated to within

three or four minutes, when the compass is mounted on a stand. On looking through the slit at A, the eye, by means of a triangular prism, sees at the same time the thread, B, and the compass-card, in such a manner as to make the divisions on the card seem a continuation of the thread; and the division with which the thread coincides, when the needle is at rest, is the magnetic azimuth of whatever object the thread may bisect. A hinge-joint connects the prism with the box, and enables it to be turned over in a convenient position to fit into the case. The sight-vane, B, has a fine thread stretched along its opening in the direction of its length, which is brought to bisect any object by turning the box round horizontally; the vane also turns on a hinge-joint, and can be laid flat upon the box for the convenience of carriage. A little knob (not seen in the figure) touches a spring, by which the vibrations of the card are checked for speedier adjustment to an object; and C is a little lever by which the card is thrown off its centre; which should always be done when the instrument is not in use, as the constant playing of the needle would wear the point upon which it is balanced, and upon the fineness of the point much of the accuracy of the instrument depends. The milled head, D, fixes the compass, when required. There is a cover to the box, which is about three inches in diameter, and one in depth.

The method of using the instrument is very simple: Having fixed it on the stand, place it immediately over the station-point, spreading the legs so as to give sufficient firmness; observe that the card is level, or nearly so, in order that it may play freely; then raise the prism, by means of the slide at E, until the divisions of the compasscard are distinctly seen; now look through the slit, A, and turn the box round, until the thread, B, bisects the object you are observing; allow the card to settle, and the

division on the card which coincides with the thread of the vane will be the azimuth, or bearing of the object, reckoned from the north or south point of the needle, when the card is divided into twice 180°, as I recommend.

The angular distance between any two objects will, of course, be the difference of their bearings; thus, suppose one to bear 15° N.E., and the other 165° S.E., the angular distance between them will be 150°.

In military sketching, the compass is often supported by the hands, when the little spring, to check the vibrations of the card, is very useful. In windy weather it is necessary to watch these vibrations, and adopt the mean as the bearing sought. When held in the hand, this instrument will give a bearing to within about 15 minutes: but if there be much wind, it will not be nearer, perhaps, than half a degree of the truth.

Dark glasses are sometimes fixed to the prism-case, and a mirror also to the sight-vane: these are used when taking azimuths of the sun. They add to the cost of the instrument; and, being seldom required, I recommend officers to dispense with them, when they want a compass for ordinary surveying purposes.

SECTION XIV.

RESUME ON SURVEYING INSTRUMENTS-OF THE MERIDIAN

LINE.

HAVING given a sufficiently minute description of the instruments commonly used by military men, and explained their several adjustments, it only remains for me to point out those among them that will be found most generally useful.

With regard to the theodolite, which is unquestionably the best instrument employed in surveying, it may be observed, that an officer seldom finds it necessary to provide himself with one; as, whenever he is employed on an extensive survey, proper instruments will be furnished him for the duty besides, the theodolite is much too cumbersome an instrument to form any part of his personal equipment. Field instruments, as the compass, sextant, &c., he must have of his own.

I should imagine that, in the course of the preceding pages, it has been sufficiently shown, that a good prismatic compass is adequate to every purpose of ordinary military sketching. With a tripod stand, which is essential, we are enabled to obtain bearings with considerable accuracy; at least, they will be near enough to the truth for any ordinary military purpose.

The result of our observations goes to show, that a person wishing to limit himself to a single instrument, will do well to select the compass in preference to any other.

The box-sextant is an excellent little instrument, as I think we have proved; but, unlike the compass, it will not do alone. With it we cannot lay down a meridian line, nor can we find our place on a plan, by knowing the situations of two others; neither can we use it for surveying roads, rivers, &c.: all objects of absolute necessity in military sketching.

I shall say nothing here of the reflecting semicircle, as the remarks on the sextant apply equally to that in

strument.

Instrument-makers have various additions and improvements both to the compass and sextant; but I recommend them to be in their simplest form: I would not even have any contrivance whatver for levelling the prismatic compass, which is easily effected by moving the legs of the stand. The expensive toys that adorn some of the opticians' shops, with all their complicated machinery, are only worthy to be considered as curiosities for the amusement of fanciful amateurs. I have never seen a practical man use any but simple surveying instruments. It is a maxim, that engines and machines for military service, should always be simple in their construction, and easily repaired when put out of order. Our surveying instruments form no exception to the rule.

OF THE MERIDIAN LINE.

All surveys should have a true meridian laid down on them, as well as the magnetic one; which last is variable. Any person using a map or plan, immediately begins to refer all imaginary lines connecting towns or villages, as well as the directions of roads, rivers, ridges of hills, &c., to the cardinal points this is his first step towards an acquaintance with the country by means of a map.