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the eye-glass first, and the object-glass afterwards. The setting of the object-glass, by introducing more distant rays of light, will affect the focus of the eye-glass, and produce parallax or indistinctness of the wires, when there was none before; the eye-piece must, in this case, be adjusted again.

Generally, when once set for the day, there is no occasion for altering the eye-glass, but the object-glass will of course have to be altered at every change of distance of the object.

In adjusting the instrument, the parallax should be first corrected, and then the error of collimation. The line of collimation being thus brought to coincide with the axis of the tube of the telescope, two further adjustments are necessary: the first to adjust the bubble-tube, so that it may truly indicate when the axis of the telescope is horizontal; and the second to set the axis of the telescope perpendicular to the vertical axis round which the instrument turns.

To adjust the Bubble-Tube.-Move the telescope till it lies in the direction of two of the parallel plate screws, and by giving motion to these screws bring the air bubble to the center of its run. Now reverse the telescope carefully in the Ys, that is, turn it end for end; and, should the bubble not settle at the same point of the tube as before, it shows that the bubble-tube is out of adjustment, and requires correcting. The end to which the bubble retires must then be noticed, and the bubble made to return one-half the distance by turning the parallel plate screws, and the other half by turning the capstan-headed screw at the end of the bubble-tube. The telescope must now again be reversed, and the operation be repeated, until the bubble settles at the same point of the tube, in the center of its run, in both positions of the instrument.

The adjustment is then perfect, and the clips which serve to confine the telescope in the Ys should be made fast. Lastly, to set the Axis of the Telescope perpendicular to the Vertical Axis round which the Instrument turns.-Place the telescope over two of the parallel plate screws, and move them, unscrewing one while screwing up the other, until the bubble of the level settles in the center of its run; then turn the instrument half round upon the vertical axis, so that the contrary ends of the telescope may be over the same two screws, and, if the bubble does not again settle at the same point as before, half the error must be corrected by turning the screw B, and the other half by turning the two parallel plate screws, over which the telescope is placed. Next turn the telescope a quarter round, that it may lie over the other two screws,

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and repeat the process to bring these two screws also into adjustment; and when, after a few trials, the bubble maintains exactly the same position in the center of its run, while the telescope is turned all round upon the axis, this axis will be truly vertical, and the axis of the telescope, being horizontal by reason of the previous adjustment of the bubbletube, will be perpendicular to that vertical axis, and remain truly horizontal, while the telescope is turned completely round upon the staves. The adjustment is therefore perfect.

The object of the above adjustments is to make the line of collimation move round in a horizontal plane, when the instrument is turned round its vertical axis, and the methods above explained suppose that the telescope itself is constructed with the utmost perfection, so that the axis of the tube carry ing the object-glass is always in the same straight line with the axis of the main tube, which carries the diaphragm with the cross wires. If this perfection in the construction of the instrument does not exist, the line of collimation will vary, as the tube carrying the object-glass is thrust out, and drawn in, to adjust the focus for objects of different distances. What is really required, then, is that the cross wires be so adjusted that the line of collimation may be in the same straight line with the line in which the center of the object-glass is moved, and that the bubble of the level be at the center of its run, when this line of collimation is directed to view objects, at the same level, or at the same distance from the center of the earth

We are indebted to Mr. Gravatt, of whose level we shall hereafter speak, for a method of collimating, which satisfies the above requirements, and removes any error arising from imperfection in the slide of the telescope, while at the same time the line of collimation is set with the end at the objectglass, slightly depressed, instead of exactly horizontal, so as to remove, or nearly so, the errors arising from the curvature of the earth, and the horizontal refraction.

To examine and correct the Collimation by Mr. Gravatt's Method." On a tolerably level piece of ground drive in three stakes at intervals of about four or five chains, calling the first stake a, the second b, and the third c.

"Place the instrument half way between the stakes a and b, and read the staff A, placed on the stake a, and also the staff B, placed on the stake b; call the two readings, Aʼand B'; then, although the instrument be out of adjustment *,

*The axis of the instrument is to be set vertical by means of the parallel plate screws, by placing the telescope over each pair alternately, and moving them, until the air bubble remains in the same position, when the instru ment is turned half round upon its axis.

yet the points read off will be equidistant from the earth's center, and consequently level.

"Now remove the instrument to a point half way between b and c. Again read off the staff B, and read also a staff placed on the stake c, which call staff c (the one before called a being removed into that situation). Now, by adding the difference of the readings on B (with its proper sign) to the reading on c, we get three points, say A', B', and c', equidistant from the earth's center, or in the same true level.

"Place the instrument at any short distance, say half a chain beyond it, and, using the bubble merely to see that you do not disturb the instrument, read all three staffs, or, to speak more correctly, get a reading from each of the stakes, a, b, c; call these three readings A", B', d'. Now, if the stake b be half way between a and c *, then ought o''-c'-(A"-A') to be equal to 2 [B"-B′-(A"-A)]; but if not, alter the screws which adjust the diaphragm, and consequently the horizontal spider line, or wire, until such be the case; and then the instrument will be adjusted for collimation.

"To adjust the spirit bubble without removing the instrument, read the staff A, say it reads A then adding (A"-A) with its proper sign to B' we

get a value, say B'".

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"Adjust the instrument by means of the parallel plate screwst, to read B"" on the staff B.

"Now, by the screws attached to the bubble-tube, bring the bubble into the center of its run.

"The instrument will now be in complete practical adjustment for level, curvature, and horizontal refraction, for any distance not exceeding ten chains, the maximum error being onlyth of a foot."

Before making observations with this instrument, the adjustments should be carefully examined and rectified, after which the screw в should never be touched; but at each station the parallel plate screws alone should be used for setting the axis round which the instrument turns truly vertical, when, in consequence of the adjustments previously made, the line of collimation will be truly level. For this purpose the telescope must be placed over each pair of the parallel plate screws alternately, and they must be moved till the air bubble settles in the middle of the level, and the operation being repeated till the telescope can be turned quite round upon the staff-head, without any change taking place in the position of the bubble, the instrument will be ready to read off the graduations upon the levelling staves, which we proceed to describe.

* Whatever be the distances between the stakes a, b, and c, the following proportions ought to hold, viz. :—

The distance from a : b: the distance a to c :: B"--B′ — (A′′ — A') : α′′ — d —(A”—A').

If this adjustment be made by the screw B, instead of the parallel plate screws, the line of collimation will be brought into its proper position with respect to the vertical axis.

The best constructed levelling staff* consists of three parts, which pack together for carriage in a neat manner, and, when opened out for use, form a staff seventeen feet long, jointed together something after the manner of a fishing-rod. The whole length is divided into hundredths of a foot, alternately coloured black and white, and occupying half the breadth of the staff; but for distinctness the lines denoting tenths of feet are continued the whole breadth, every half foot or five-tenths being distinguished by a conspicuous black dot on each side.

In all work where great accuracy is required, the Y level, above described, is preferable to either of the others; but both Troughton's level and Gravatt's level are calculated, by their lightness, and by their being less liable to derangement when once properly adjusted, to get rapidly over the ground

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In this level the telescope, T, rests close down upon the horizontal bar, bb, the spirit level, ll, is permanently fixed to the top of the telescope, and does not, therefore, admit of adjustment, and the compass box, c, is supported over the level by four small pillars attached to the horizontal bar. This construction makes the instrument very firm and compact. The staves, staff-head, and parallel plates by which the instrument is supported, and the vertical axis upon which it turns, are of exactly the same construction as has been already described as used for supporting the Y level.

The diaphragm is furnished with three threads, two of them vertical, between which the levelling staff may be seen, and the third, horizontal, gives the reading of the staff by its coincidence with one of the graduations marked upon it. Sometimes a pearl micrometer-scale is fixed on the diaphragm, instead of the wires. The central division on the scale, then, indicates the collimating point, and by its coincidence with a division of the levelling staff gives the required reading of

This staff was first introduced into use by William Gravatt, Esq.

this staff; and the scale serves the purpose of measuring distances approximately, and of determining stations nearly equidistant from the instrument, since at such equal distances the staff will subtend the same number of divisions upon the micrometer-scale.

In selecting a level of Troughton's construction, and also in testing and adjusting the collimation subsequently, Mr. Gravatt's method, already described, is the best to be used; and, when the line of collimation is thus brought into adjustment, if the bubble be far from the center of its run, the fault can only be remedied by the maker; but, if the bubble settle very nearly in the center of its run, the instrument may be deemed a good one, and, the divisions on the glass tube which coincide with the ends of the bubble being noted, the instrument must be set up for use with the bubble in this position.

The line of collimation is set perpendicular to the vertical axis, in the same manner as in the Y level, by means of the capstan screws, BB, the bubble being made to maintain the requisite position, as above determined, while the instrument is turned completely round on its axis.

MR. GRAVATT'S LEVEL.

This instrument is furnished with an object-glass of large aperture and short focal length; and, sufficient light being

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thus obtained to admit of a higher magnifying power in the eye-piece, the advantages of a much larger instrument are obtained, without the inconvenience of its length. The diaphragm is carried by the internal tube a a, which is nearly equal in length to the external tube. The external tube T T is sprung at its aperture, and gives a steady and even motion to the internal tube a a, which is thrust out, and drawn in, to adjust the focus for objects at different distances by means of the milled-headed screw A. The spirit level is placed above the telescope, and attached to it by capstan-headed screws,

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