V. The breadth of a river, or other short distance, may be taken thus:—take two pickets of different lengths, drive the shortest into the ground close to the edge of the bank; measure some paces back from it, and drive in the other till you find, by looking over the tops of both, that your sight cuts the opposite side.—Then pull up the first picket, measure the same distance from the second, in any direction the most horizontal, and drive it as deep in the ground as before.—Consequently, if you look over them again, and observe where the line of sight falls, you will have the distance required. TEILK. VI. The following simple method of ascertaining the breadth of a river may be sufficiently correct for some cases: Place yourself at the edge of one bank, and lower one corner of your hat till you find the edge of it cuts the other bank; then steady your head, by placing your hand under your chin, and turn gently round to some level spot of ground, and observe where your eyes and the edge of the hat again meet the ground; your distance from that point will be nearly the breadth of the river. TEILK. VII. Distances ascertained by the difference between the true and apparent level. See _Levelling_. VIII. Distances measured by sound. See _Sound_. IX. The following simple micrometer may be so usefully applied to military purposes, that I shall extract it verbatim from the Philosophical Transactions for 1791, where it is described by Cavallo. This micrometer consists of a thin and narrow slip of mother of pearl, finely divided, and placed in the _focus_ of the eye glass of a telescope, just where the image is formed.—It is immaterial whether the telescope be a reflector, or a refractor, provided the eye glass be a _convex_ lens and not a _concave_ one, as in the Galilean construction.—The simplest way to fix it, is to stick it on the diaphragm, which generally stands within the tube, and in the focus of the eye glass.—When thus fixed, if you look through the eye glass, the divisions on the scale will appear very distinct, unless the diaphragm is not exactly in the focus; in which case the scale must be placed exactly in the focus, by pushing the diaphragm, backwards or forwards, when this is practicable; or else the scale may be easily removed from one surface of the diaphragm to the other, by the interposition of a circular bit of paper or card, or a piece of sealing wax.—This construction is fully sufficient when the telescope is always to be used by the same person; but when different persons are to use it, then the diaphragm, which supports the micrometer, must be so constructed as to be easily moved backwards or forwards, though that motion need not be greater than about the tenth or eighth of an inch. This is necessary, because the distance of the focus of the same lens appears different to the eyes of different persons; and therefore whoever is going to use the telescope for the mensuration of an angle, must first