The instrument above described is of moderate dimen• sions, the diameter of the vertical circle being 18 inches, and that of the horizontal one 15 inches. They are both divided by lines (not dots) into spaces of five minutes, and the parts actually divided are, by their respective micrometers, subdivided and read, as was said above, to every second. The essential part of those microme ters consists of two wires, which cross each other like an X, and are moved along by a fine screw, in the direc tion of a tangent, to that part of the circle which may be opposed to them. When the cross is at the zero of its scale, it is the index of the divisions of the circle, and the quantity to be measured by the motion of the cross is that by which the next dividing line has passed beyond that zero, and is always additive. The general con struction of this micrometer is not materially different from the parallel-line one, which was described in our article ASTRONOMY.
The length of the upper telescope is 30 inches, the aperture of its object-glass .2 inches, and its magnifying power about 60. It is indeed furnished with eye-pieces of both less and greater power than this, which may have their use in particular cases ; but in general, this is reckoned the most useful power for such a telescope. The lower telescope is only about 18 inches in length, and its aperture and power proportionable. The whole height of the instrument is something more than three feet, and its weight is about 75 pounds.
In the eye-piece of the upper telescope are placed five vertical and five horizontal lines, the angular dis tance of which is about 6' 24" of a degree ; this distance, when the sun's diameter is 32 minutes, allows an equal portion of time between all the ten contacts in observing his transit, or passage over the meridian. The five horizontal lines arc for the purpose of observing equal altitudes, and when the diameter of that luminary is as above, will, without moving the circle, afford ten distinct sights, and these also will be equal to each other.
The lower telescope has in its focus only two cross w fires. It is never elevated to the heavens ; its only purpose being to keep watch, so as to shew if the whole instrument preserves its position unvaried respecting some distant object, while a set of observations is car ried on.
In the eye-end of the upper telescope there are several adjustments. One of these is for placing the cross-lines in the focal point of the object-glass ; and this is effected by drawing out or pushing in the whole of the eye-piece respecting the main tube. The whole eye-piece also ad mits of a limited motion round the axis of vision, where by the cross-lines are adjusted respecting their vertical and horizontal position. The plate in which the cross lines are inserted, is acted on horizontally by two oppo site screws ; they draw, and produce that adjustment, which places the intersection of the middle lines per pendicular to the horizontal axis. When this is verified
by reversing the circle upon its angular supports, the line of collimation of the telescope will describe a great circle round the axis on which it revolves.
The cross-lines of the lower telescope are also acted on by two screws, like the upper one ; but these are for no other purpose than for bisecting an object with the greatest accuracy, which cannot be dune by the motions of the telescope itself, because they are nut governed by adjusting screws.
To proceed in our account of the adjustments of this instrument, we find the feet-screws ready for placing the ertical axis truly perpendicular. This they will do by help of the level X, which having its bubble brought to the divided scales in any position, and then the instru ment being turned half round in azimuth, an error appear the double of that by which the axis is inclined ; one half therefore mmt be corrected by the feet-strews, and the other by the screw r which acts upon the level. This will set the axis perpendicular in the direction of the level ; and now, the instrument being turned through a quadrant, and the level brought by the feet-screw again to the sliders, this adjustment will be effected. The first part of the operation is best performed with the level in the direction of two of the feet-screws, be cause the second may then be made by the other screw without danger of deranging the first. It need scarcely be mentioned, that the operation should be repeated for greater exactness ; nor that, when tl.e adjustment is perfect, the bubble of the level will preserve its cen tricabposition, while the instrument is turned quite round in azimuth. By this adjustment, the telescope is made to describe lines parallel to the horizon. The reading microscopes may now be adjusted to their places, so as to have their zeros exactly in the diameter of the azimuth circle.
The next great adjustment is to place the vertical circle at such a height, that its two microscopes shall be directed to two opposite points in its horizontal diame ter. This is clone by the screw f, which, acting upon the angular bearings of the horizontal axis, carries it up or down, as may be required. The farther pillar has the same kind of screw, which acts in like manner upon the support of the opposite end of the axis. It must here be mentioned, that through an inadvertence of the maker, the draftsman had access to the instrument, at a time when the angular bearings d were screwed almost out of their sockets e, on which account the socket and friction-wheel g appears to lift the circle too high ; and the draftsman, knowing that the microscopes should oc cupy the line of the horizontal diameter of the circle, added that part in the tubular support which is seen be tween the circular rings, which part was not to be found in the instrument.