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put in with great rapidity by means of the instrument which we are now about to describe

A A, B B, C C, are three arms moveable about a cominon center, o, and carrying three fine wires stretched quite tight, the prolongation of which would pass exactly through the center, o. The arms, B B, and c c, carry each a graduated arc, bb, and c c; and the arm AA carries two verniers, a, a', adapted to the graduated arcs, b b, and c c, respectively.

M

The angles L O M, and L O N, subtended, at one of the points which we wish to put in, by L, M, and N, three of the principal points of the survey already laid down, having been observed by means of the prismatic compass, or pocket sextant, the arms of the station pointer are opened out till the verniers point out these angles upon the graduated limbs, bb, and c c, respectively. The station pointer being then placed upon the paper, and moved about till the fine wires pass exactly over the stations L, M, N, as marked already upon the plot, the center will be exactly over the point to be filled in, and its place is to be marked by passing a pricking point through a small opening, which is made at the center to serve this purpose.

PART IV.-ON ASTRONOMICAL INSTRUMENTS.

THE space which we have occupied with the former parts f this work will compel us to be more brief than we had contemplated in the description of the Astronomical Instruments. We shall, therefore, confine our attention more particularly to Hadley's Sextant;

Troughton's Reflecting Circle;

The Transit Instrument;

The Collimator;

Altitude and Azimuth Instruments;

with some brief remarks upon other instruments, used for astronomical purposes.

HADLEY'S SEXTANT.

This instrument differs from the pocket sextant, already described, in its appearance, from the absence of the box in which the pocket sextant is fixed; in its size, varying usually from 4 inches to 6 inches radius; and in its requiring and admitting of more perfect and minute adjustment.

L is the graduated limb of the instrument, graduated from 0° to 140° at every 10' or 20', according to the size of the instrument, and subdivided by the vernier, v, to 10" or 20",

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thus enabling us to read off angles by estimation to 5". The limb is also graduated through a small space, called the arc of excess, on the other side of the zero point. T is the tangent screw for giving a slow motion to the index bar after it has been clamped by the screw c. м is a microscope, attached to the index bar moveable by a screw, v, so as to command a view of the vernier throughout its entire length.

I is the index glass, or first reflector, attached to and moving with the index bar; and H is the horizon glass, having its lower half silvered to form the second reflector, and its upper half transparent. Four dark glasses are placed at c, any one or more of which can be turned down between the index glass

and horizon glass to moderate the intensity of the light from any very bright object viewed by reflection; and at G are three dark glasses, any one or more of which can be turned up to moderate the intensity of the light from any bright object, viewed directly through the transparent part of the horizon glass. D is a ring for carrying the telescope, attached to a stem s, called the up and down piece, which can be raised or lowered by means of a milled-headed screw. The use of this up and down piece is to raise or lower the telescope, till the objects seen directly and by reflection appear of the same brightness. K is the handle by which the instrument is held.

In selecting an instrument care must be taken that all the joints of the frame are close, without the least opening or looseness, and that all the screws act well, and remain steady, while the instrument is shaken by being carried from place to place. All the divisions on the limb and vernier, when viewed through the microscope, must appear exceedingly fine and distinct, and the inlaid plates, upon which the divisions are marked, must be perfectly level with the surface of the instrument. The index, or zero, of the vernier, should also be brought into exact coincidence successively with each division of the limb, till the last division upon the vernier reaches the last division upon the limb; and, if the last division of the vernier do not in each case also exactly coincide with a division upon the limb, the instrument is badly graduated, and should be rejected. All the glass used in the instrument should be of the best quality, and the glasses of the reflectors should each have their faces ground and polished perfectly parallel to each other, to avoid refraction. Look, therefore, into each reflector, separately, in a very oblique direction, and observe the image of some distant object; and if the image appears clear and distinct in every part of the reflector, the glass is of good quality; but if the image appears notched, or drawn with small lines, the glass is veiny, and must be rejected. Again, if the image appears singly, and well defined about the edges, the two surfaces of the glass are truly parallel; but if the edge of the image appears misty, or separated like two images, the two surfaces are inclined to one another. The examination will be more perfect if the image be examined with a small telescope.

A plain tube and two telescopes, one showing objects inverted, and the other erect, are usually supplied with the sextant. The manner of testing the telescopes has already been explained in the part of the work devoted to optical in

struments. A dark glass is also supplied to fit on to the eyeend of the telescope, and a key for turning the adjusting

screws.

To examine the Error arising from the Imperfection of the Dark Glasses.-Fit the dark glass to the eye-end of the telescope, and, all the shades being removed, bring the reflected image of the sun into contact with its image seen directly through the unsilvered part of the horizon glass. Then remove the dark glass from the eye-end of the telescope, and, setting up first each shade separately, and then their various combinations, if the two images do not in any case remain in contact, the angle through which the index must be moved to restore the contact, is the error of the dark glass, or combination of dark glasses, used in the observation, and which error should be recorded for each glass and each combination of the glasses.

The adjustments of the instrument consist in setting the horizon glass perpendicular to the plane of the instrument, and in setting the line of collimation of the telescope parallel to the plane of the instrument.

To adjust the Horizon Glass.-While looking steadily at any convenient object, sweep the index slowly along the limb, and, if the reflected image do not pass exactly over the direct image, but one projects laterally beyond the other, then the reflectors are not both perpendicular to the face of the limb Now the index glass is fixed in its place by the maker, and generally remains perpendicular to the plane of the instru ment, and, if it be correctly so, the horizon glass is adjusted by turning a small screw at the bottom of the frame in which it is set, till the reflected image passes exactly over the direct image

To examine if the Index Glass be perpendicular to the Plane of the Instrument.-Bring the vernier to indicate about 45°, and look obliquely into this mirror, so as to view the sharp edge of the limb of the instrument by direct vision to the right hand, and by reflection to the left. If, then, the edge and its image appear as one continued arc of a circle, the index glass is correctly perpendicular to the plane of the instrument; but if the arc appears broken, the instrument must be sent to the maker to have the index glass adjusted.

To adjust the Line of Collimation.—1. Fix the telescope in its place and turn the eye-tube round, that the wires in the focus of the eye-glass may be parallel to the plane of the instrument. 2. Move the index till two objects, as the sun and moon, or the moon and a star, more than 90° distant from each other, are brought into contact at the wire of the dia

phragm, which is nearest the plane of the instrument. 3. Now fix the index, and altering slightly the position of the instrument, cause the objects to appear on the other wire; and, if the contact still remain perfect, the line of collimation is in correct adjustment. If, however, the two objects appear to separate at the wire that is further from the plane of the instrument, the object-end of the telescope inclines towards the plane of the instrument; but, if they overlap, then the objectend of the telescope declines from the plane of the instrument. In either case the correct adjustment is to be obtained by means of the two screws, which fasten to the up and down piece the collar holding the telescope, tightening one screw and turning back the other, till, after a few trials, the contact remains perfect at both wires.

The instrument having been found by the preceding methods to be in perfect adjustment, set the index to zero, and if the direct and reflected images of any object do not perfectly coincide, the arc, through which the index has to be moved to bring them into perfect coincidence, constitutes what is called the index error, which must be applied to all observed angles as a constant correction.

To determine the Index Error.—The most approved method is to measure the sun's diameter, both on the arc of the instrument, properly so called, to the left of the zero of the limb, and on the arc of excess to the right of the zero of the limb. For this purpose, firstly, clamp the index at about 30' to the left of zero, and, looking at the sun, bring the reflected image of his upper limb into contact with the direct image of his lower limb, by turning the tangent screw, and set down the minutes and seconds denoted by the vernier; secondly, clamp the index at about 30' to the right of zero, on the arc of excess, and, looking at the sun, bring the reflected image of his lower limb into contact with the direct image of his upper limb, by turning the tangent screw, and set down the minutes and seconds denoted by the vernier underneath the reading before set down. Then half the sum of these two readings will be the correct diameter of the sun, and half their difference will be the index error. When the reading on the arc of excess is the greater of the two, the index error, thus found, must be added to all the readings of the instrument; and when the reading on the arc of excess is the less, the index error must be subtracted in all cases. To obtain the index error with the greatest accuracy, it is best to repeat the above operation several times, obtaining several readings on the arc of the instrument, and the same number on the arc of excess; and

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