the underside of the plate, this pin is connected with a ball that fits into a socket in the lower plate; the axis of the pin and ball are so framed, as to be always perpendicular to the plate, and, consequently, to the horizontal limb.

To adjust the theodolite. As so much of surveying depends on the accuracy of the instruments, it is ab-. solutely necessary that the surveyor should be very expert in their adjustments, without which he cannot expect the instruments will properly answer the purposes they were designed for, or that his surveys will have the requisite exactness.

The necessary adjustments to the theodolite, which we have just described, are, 1. That the line of sight, or collimation, be exactly in the centre of the cylindric rings round the telescope, and which lie in the Y's. 2. That the level be parallel to this line, or the axis of the above-mentioned rings. 3. The horizontal limb must be so set, that when the vertical are is at zero, or o, and the upper part moved round, the bubble of the level will remain in the middle of the

open space.

Previous to the adjustments, place the instrument upon the staff, and set the legs thereof firmly upon the ground, and at about three feet from each other, so that the telescope may be at a proper height for the eye, and that two of the screws on the staff that are opposite to each other may be nearly in the direction of some conspicuous and distant object.

To adjust the line of collimation. Having set up the theodolite agreeable to the foregoing direction, direct the telescope to some distant object, placing it 50 that the horizontal hair, or wire, may exactly coincide with some well defined part of the object; turn the telescope, that is, so that the tube of the spirit level D may be uppermost, and observe whether the horizontal hair still coincides with the object; if it does, the hair is in its right position; if not, correct half the difference by moving the hair, Q

or wire, which motion is effected by easing one of the screws in the eye tube, and tightening the other; then turn the telescope round to its former position, with the tube of the spirit level lowermost, and make the hair coincide with the object, by moving the vertical arc; reverse the telescope again, and if the hair does not coincide with the same part of the object, you must repeat the foregoing operation, till in both positions it perfectly coincides with the same part of the object.

The precise situation of the horizontal hair being thus ascertained, adjust the vertical hair in the same manner, laying it for this purpose in an horizontal position the spirit tube will, during the adjustment of the vertical hair, be at right angles to its former position. When the two wires are thus adjusted, their intersection will coincide exactly with the same point of the object, while the telescope is turned quite round; and the hairs are not properly adjusted, till this is effected.

Adjustment of the level. To render the level parallel to the line of collimation, place the vertical arc over one pair of the staff screws, then raise one of the screws, and depress the other, till the bubble of the level is stationary in the middle of the glass; now take the telescope out of the Y's, and turn it end for end, that is, let the eye end lay where the object end was placed; and if, when in this situation, the bubble remains in the middle as before, the level is well adjusted; if it does not, that end to which the bubble runs is too high; the position thereof must be corrected by turning with a screw-driver one or both of the screws which pass through the end of the tube, till the bubble has moved half the distance it ought to come to reach the middle, and cause it to move the other half by turning the staff screws. Return the telescope to its former position, and if the adjustments have been well made, the bubble will remain in the middle; if otherwise, the process of

altering the level and the staff screws, with the reversing, must be repeated, till it bears this proof of its accuracy. In some instruments there is a provision for raising or lowering the Y's a small degree, in order more conveniently to make the bubble continue in its place when the vertical arc is at o, and the horizontal limb turn round.

To adjust the level of the horizontal limb. Place the level so that it may be in a line with two of the staff screws, then adjust it, or cause the bubble to become stationary in the middle of the open space by means of these screws. Turn the horizontal limb half round, and if the bubble remains in the middle as before, the level is well adjusted; if not, correct half the error by the screws at the end of the level, and the other half by the staff screws. Now return the horizontal limb to its former position, and if it remains in the middle, the errors are corrected; if not, the process of altering must be pursued till the error is annihilated. See this adjustment in the description of Ramsden's theodolite.

When the bubble is adjusted, the horizontal limb may always be levelled by means of the staff

screws.

OF THE THEODOLITE, AS IMPROVED BY RAMSDEN,

fig. 2, plate 16.

Among the improvements the intruments of science have received from Mr. Ramsden, and the perfection with which he has constructed them, we are to rank those of the theodolite; in the present instance, he has happily combined elegance and neatness of form, with accuracy of construction; and the surveyor will contemplate with pleasure this instrument, and the various methods by which the parts concur to give the most accurate result.

The principal parts of this instrument are however

or wire, which motion is effected by easing one of the screws in the eye tube, and tightening the other; then turn the telescope round to its former position, with the tube of the spirit level lowermost, and make the hair coincide with the object, by moving the vertical arc; reverse the telescope again, and if the hair does not coincide with the same part of the object, you must repeat the foregoing operation, till in both positions it perfectly coincides with the same part of the object.

The precise situation of the horizontal hair being thus ascertained, adjust the vertical hair in the same manner, laying it for this purpose in an horizontal position: the spirit tube will, during the adjustment of the vertical hair, be at right angles to its former position. When the two wires are thus adjusted, their intersection will coincide exactly with the same point of the object, while the telescope is turned quite round; and the hairs are not properly adjusted, till this is effected.

Adjustment of the level. To render the level parallel to the line of collimation, place the vertical arc over one pair of the staff screws, then raise one of the screws, and depress the other, till the bubble of the level is stationary in the middle of the glass; now take the telescope out of the Y's, and turn it end for end, that is, let the eye end lay where the object end was placed; and if, when in this situation, the bubble remains in the middle as before, the level is well adjusted; if it does not, that end to which the bubble runs is too high; the position thereof must be corrected by turning with a screw-driver one or both of the screws which pass through the end of the tube, till the bubble has moved half the distance it ought to come to reach the middle, and cause it to move the other half by turning the staff screws. Return the telescope to its former position, and if the adjustments have been well made, the bubble will remain in the middle; if otherwise, the process of

its

altering the level and the staff screws, with the reversing, must be repeated, till it bears this proof of accuracy. In some instruments there is a provision for raising or lowering the Y's a small degree, in order more conveniently to make the bubble continue in its place when the vertical arc is at o, and the horizontal limb turn round.

To adjust the level of the horizontal limb. Place the level so that it may be in a line with two of the staff screws, then adjust it, or cause the bubble to become stationary in the middle of the open space by means of these screws. Turn the horizontal limb half round, and if the bubble remains in the middle as before, the level is well adjusted; if not, correct half the error by the screws at the end of the level, and the other half by the staff screws. Now return the horizontal limb to its former position, and if it remains in the middle, the errors are corrected; if not, the process of altering must be pursued till the error is annihilated. See this adjustment in the description of Ramsden's theodolite.

When the bubble is adjusted, the horizontal limb may always be levelled by means of the staff

screws.

OF THE THEODOLITE, AS IMPROVED BY RAMSDEN, fig. 2, plate 16.

Among the improvements the intruments of science have received from Mr. Ramsden, and the perfection with which he has constructed them, we are to rank those of the theodolite; in the present instance, he has happily combined elegance and neatness of form, with accuracy of construction; and the surveyor will contemplate with pleasure this instrument, and the various methods by which the parts concur to give the most accurate result.

The principal parts of this instrument are however