for the sound to travel, he can approximate very closely to the distance between the observers.

ESTIMATION.-The most inaccurate, yet the most generally used, method of measuring distances for filling in details in hasty sketching is that of estimation by the eye. The art of quite accurately estimating distances can be acquired by careful practice, and it is of the greatest importance to the sketcher, as most distances up to 100 yards are estimated. This is done by a mental comparison with certain known distances, or it may be done on the principle of similar triangles by holding a rule at a certain known distance in front of the eye and seeing how great a space on the rule the object covers; from this its distance may be deduced, provided the size of the object be known.

The degree of approximation attainable after proper practice in the estimation of distances precludes the probability of an error of more than 10 per cent up to 300 yards, of more than 12 per cent up to 600 yards, of more than 16 per cent up to 1200 yards.

In determining distances by sight the estimate is based upon the distinctness with which the object can be seen, upon its apparent height when its dimensions are known, and upon a comparison of the extent of the ground between it and the observer with some other known distance which is either within view or so distinctly impressed upon one's memory as to serve accurately as a unit of measure. The distinctness with which any object at any particular distance is visible varies considerably with different men; hence no inflexible rule can be expressed. Actual practice must determine for every individual. As the atmospheric conditions and nature of the background greatly affect the degree of visibility of objects, the practice should be conducted in different varieties of weather and along lines variously situated with reference to the sun and any surrounding hills or woods, particular attention being paid to all these circumstances.

When the light shines directly on objects, or when they are light-colored, or when they are seen against a light background, their details are more clearly visible, and they appear nearer than they really are. So also if the observer's back be towards the sun, or the observation be made in winter when the air is dry and clear, or else just before or after a rain, or if the ground be level and of a uniform tint, or if it rise towards the object, the distance will appear less than it really is.

Under the reverse conditions the distance will appear greater. The tendency in looking for an elevation down to a lower level is to over-estimate the true distance and overestimate the degree of slope, while in looking upward to a height it is just the reverse. On a wide plain of uniform color, if the eye be arrested by no intermediate points, the estimate will be generally too short.

CHAPTER XVI.

MEASUREMENT OF DIRECTIONS.

In Topographic Sketching the instrument ordinarily used to measure directions is some form of hand compass.

THE BOX COMPASS.-Description. The rectangular box compass consists of a circular brass box, from 2 to 3 inches in diameter, containing a needle and graduated circle or card, sunken its depth in a rectangular block of wood from 2 to 3/4 inches thick, with a hinged cover. To prevent unnecessary wear, a lever and pin are so arranged that the needle is lifted from its pivot whenever the lid is closed.

for lifting the needle is also provided.

The card or ring is graduated into 360°, from 0° to

the right 90°, etc., clockwise (Fig. 197), or from 0° to the left 90°, etc., contra-clockwise (Fig. 198). In one pattern the 0° -180° line is parallel to the hinged edge of the box. An edge of the lid, when raised at right angles to the block, is used for the line of sight. In another (Fig. 196) the line of sight is at right-angles to the hinged edge. A mark on the lid in prolongation of the 0°-180° line being used as the line of sight.

Use. To take a bearing with the first-mentioned pattern, raise the lid at right angles to the box and hold the latter horizontal. The lid being generally hinged on the right-hand side of the 0°-180°, the compass is held in the right hand at several inches in front of the eye. With the zero end of the 0°-180° line directed towards the object, sight along an edge of the lid at the object, watching the oscillations of the needle, which may be checked by pressing on the pin for raising it. The needle must be free, however, at the time of settling. When it has settled, read the north end of the needle. The mean of two or three readings may be taken as the correct one. With the second pattern, hold the compass with the lid towards the object and bring the line on the lid in line with the object and its reflection on the glass over the pivot, then read the north end of the needle. In both these patterns the line of sight and graduated ring both move around the needle, which remains stationary in taking bearings; hence, when figured from 0 at the north to the left (Fig. 198), the readings as given by the north end of the needle are the angles from the magnetic meridian at the north around to east, south, and

When figured from 0 at the north to the right (Fig. 197), the readings of the north end of the needle are the angles from the magnetic meridian at the north around to the west, south, and east.

THE PRISMATIC COMPASS (Fig. 199).-Description. This compass differs from the box compass described, in that the graduated disk or card is fastened on top of the needle with the 0°-180° line coincident with it. The Oof the disk is usually

placed over the south end of the needle and the 180° division over the north end. On the south end of the box is a sight-vane with a prism which reflects the graduations on the disk up to the eye, while the eye at the same time sees the object observed through a slit in this sight, and the vertical hair in the leaf-sight on the north end. When the leaf-sight is folded down it raises the needle off the pivot. The swing of the needle is checked by pressing on a little button under the leaf-sight which presses a spring against the edge of the disk. On some prismatic compasses there is a mirror on the leaf-sight for reflecting elevated or depressed objects to the eye, and on the prism sight there are colored glasses to be interposed when observing the sun.

FIGURE 199.

Use. To take a direction with a prismatic compass, turn the prism sight up, slide the prism up or down until the graduations are distinctly seen through it; raise the front-sight; then, holding the prism sight near the eye, sight through the slit above the prism and bring the vertical hair in the leaf sight on the object, and when the needle comes to rest, read the division on the disk which the hair appears to cut. With the prismatic compass, on account of the disk being fastened on top of the needle and remaining stationary while the line of sight revolves around, the graduation of the disk under the eye is read; hence, in order to read zero when sighting north, the 0 of the disk is usually placed over the south end of the needle. If the disk is graduated from 0 clockwise (Fig. 197, page 262), the different readings will be angles from the magnetic meridian to the east, south, west, etc., but if graduated contra-clockwise (Fig. 198, page 262), the readings will be angles from the magnetic meridian to the west, south, east, etc.