adjusting screws at one end of the level tube, which regulate its lateral movement, and repeat the operation until the bubble will remain centered during the partial revolution of the telescope.

To adjust the level tube vertically, center the bubble accurately, take the telescope out of the wyes, turn it end for end, and replace it in the wyes very carefully so as not to disturb their position. If the bubble remains in the center of the tube, the adjustment is perfect. If the bubble runs to one end, bring it half way back by means of the capstan-pattern adjusting nuts at one end of the level tube, by which it can be raised or lowered, and then bring it to the middle of the tube by means of the leveling screws. Repeat the operation until the bubble will remain truly centered when the telescope is reversed in the wyes.

Third Adjustment.-Level up the instrument, using each pair of leveling screws. Having centered the bubble carefully with the telescope over one pair of leveling screws, reverse the telescope or turn it end for end over the same pair of leveling screws. If the bubble runs toward one end, bring it half way back by means of the capstan-pattern nuts at the end of the level bar; then center it perfectly with the leveling screws. Repeat the operation over each pair of leveling screws alternately until the bubble will remain perfectly centered throughout an entire horizontal revolution of the telescope.

Adjustment of the Wye-Level Cross-Hairs.-Besides the preceding adjustments, it is convenient in leveling to have the horizontal cross-hair truly horizontal so as to be able to sight with any portion of it. To test this, sight upon any sharply defined point, focusing the telescope perfectly and bringing the point exactly in range with the horizontal cross-hair near either end; that is, near the right-hand or left-hand edge of the field of view. Then, revolve the telescope slowly on its vertical axis and notice whether or not the point sighted is cut exactly the same by the cross-hair throughout its entire length. If any deviation is discernible, it should be corrected by carefully rotating the cross-hairs in a direction opposite to that in which it appears they should be rotated, until the horizontal crosshair will cut the point exactly the same throughout its length

T

when the telescope is revolved slowly on the vertical axis of the instrument.

THE DUMPY LEVEL

An engineers' dumpy level of American make is shown in the accompanying illustration. In its general construction it is similar to the wye level. The essential difference is that in the dumpy level the telescope AB is attached rigidly to the horizontal level bar CD, and the level tube EF is attached to the

level bar and is adjustable at one end and in a vertical direction only, while the other end is attached permanently by a hinge. Since the telescope cannot be revolved in its supports, there is no necessity for the lateral adjustment of the level tube.

Adjustments of the Dumpy Level.-There are two adjustments of the dumpy level, namely:

1. To make the axis of the level tube perpendicular to the vertical axis of rotation, so that the bubble will stand in the center of its scale when the telescope is revolved.

2. To make the line of sight parallel to the axis of the level tube, so that the line of sight will be horizontal when the level bubble stands in the center of its scale.

First Adjustment.-Plant the tripod firmly and level up the instrument, using each pair of leveling screws. With the telescope over one pair of leveling screws, center the bubble accurately, then reverse the telescope end for end over the same pair of leveling screws. If the bubble runs toward either end, bring it half way back by means of the capstan nuts at one end of the level tube; then center it with the leveling screws. Repeat the operation over each pair of leveling screws alternately until the bubble will remain centered perfectly throughout a complete revolution of the telescope.

Second Adjustment.-The second adjustment is effected by establishing a horizontal line and adjusting the cross-hairs to agree with it while the bubble is at the center of the tube. To establish this line, drive two pegs into the ground several hundred feet apart and determine the true difference in eleva

tion of these pegs. This can be accomplished even with an unadjusted instrument by setting it up at a place having the same distance from each peg and then taking rod readings and subtracting them. Next, set up the instrument over one peg with its center at a distance from the peg horizontally equal to about one-half the length of the telescope; bring the level bubble to the center of the tube, and with the leveling rod measure the exact height of the intersection of the cross-hairs above the peg. To determine this height on the rod, hold the graduated face of the rod about a half-inch from the eye end of the telescope, and by looking into the object end of the telescope bring the point of a pencil in the center of the small field of view on the face of the rod. Set the target at this height, plus or minus the difference in the elevations of the pegs, according as the rod reading on the distant peg was more or less than on the peg over which the instrument is set; then direct the telescope toward the rod held on the distant peg and adjust the cross-hairs so that the horizontal cross-hair will exactly bisect the target when the level bubble stands in the middle of its scale.

GENERAL PROPERTIES OF LEVELS

Sensitiveness of Level Bubble.-The sensitiveness or delicacy of the level bubble is indicated by the angle through which the line of sight must move in order to cause the bubble to move over one division of it. The smaller the angle, the more sensitive will be the bubble. The tangent of this angle can be determined by setting up the instrument and taking two rod readings at' a distance of, say, 400 ft. from the station. Take one reading when the center of the bubble is exactly at a division mark of its scale, and then by means of the leveling screws, tip the instrument just sufficiently to cause the bubble to move one division of its scale and note again the reading of the rod. If the difference of the two rod readings is r, the distance of the rod from the station is d, and the angle through which the line of sight has been moved is a, then

tan a=

r

d

Magnifying Power and Definition.-The magnifying power of a telescope is the measure of its capacity to enlarge the apparent size of an object. It is commonly expressed by the number of times greater any linear dimension of an object appears when viewed through the telescope than when viewed with the naked eye, and is commonly spoken of as the number of diameters of magnifying power.

The magnifying power of a telescope can be determined approximately in the following manner: Cut out a white card exactly .1 ft. in width and attach it to a leveling rod so as to cover exactly one of the tenth divisions; set up the rod at a distance of, say, 25 ft., direct the telescope toward the rod, and focus it perfectly. Then, by observing the rod with both eyes, but with one eye looking through the telescope, note the number of divisions on the rod, as viewed with the naked eye, that appear to be covered by the white card, as viewed through the telescope. This will be, approximately, the number of diameters of magnifying power of the telescope. It is well to repeat the observation with the other eye looking through the telescope.

The definition of a telescope indicates the degree of clearness and sharpness of outline with which objects can be seen through

it. In a general way, magnifying power and definition are opposed; that is, for the same size, a low-power telescope will ! have better definition than a high-power telescope, provided the excellence of the optical construction is the same in each

case.

It is well to note here, that for telescopes of the same length, the inverting telescope gives considerably higher magnifying power, better definition, better light, and a much more brilliant image than the erecting telescope. A well-constructed erecting telescope 18 in. long may have a magnifying power of 30 diameters, and an inverting telescope of the same length has a power of about 40 diameters.

Care of Level. The level should not be exposed to the burning rays of the sun, to rapid changes of temperature, to unequal temperatures on its different parts, or to dust, and should not be used in rainy weather when possible to avoid it. Changes of temperature disturb the adjustments, dust is injurious to the bearings and the lenses, and moisture obscures the lenses and is otherwise injurious to the instrument. Where it is impossible to avoid working in the rain, wipe the lenses frequently and carefully with a soft linen cloth, and after returning to the office or camp, wipe very carefully and thoroughly, finishing with a piece of dry chamois skin, and place in a moderately warm, dry place, so that every particle of moisture will be removed. When carrying a level on its tripod in open country, the spindle should always be clamped slightly to prevent the wearing of the centers by swinging, and the instrument should be carried with the object end of the telescope down. When working in a wooded country where underbrush is dense, the level should be carried with the spindle unclamped, so that the telescope will turn freely on the spindle and yield readily to any pressure. A blow that would inflict no injury upon an unclamped instrument might seriously damage one while clamped rigidly.

Leveling Rods.-There are two classes of leveling rods, namely, (1) rods on which the graduations are sufficiently distinct to be read directly by the leveler, and called self-reading rods, and (2) rods on which the graduations are small and which have a sliding target brought into the line of sight by