Thus much we have thought proper to say respecting the goniometer, which, although certainly not a circle, belongs to the class of repeaters, and was the original step which led to the construction of the celebrated in strument, which is the more immediate subject of our present enquiries.
When Hadley's octant became known to the world, Professor Alayer was among those who rightly apprecia ted its valuable properties, and by extending its arc to a complete circle, for the purpose of reducing the errors of division, he gave it the repeating property of his go niometer, which he bad before perhaps only thought of. The professor was fully aware. that the commd'n theo dolite, and other surveying instruments, were capable of being used zs repeaters ; and we can assign no other rea son for his giving the preference to the undivided bars, diagonal scale and compasses, than that scales must at that time have been more accurately divided than circles.
It was about the year 1789, when Borda first caused a repeating circle to be constructed for astronomical and geodetic purposes ; and one of our motives for being so particular in what is s I ant ..e. fore. to ttil.%5 exactly how much is due to him. The former repeaters, it has been seen. gave but one result for tit o operations; but by the use of an additional telescope, applied as hereafter to be described, he procured the mean. of obtaining double re sults, but this advantage is Out purchased at a less ex pence than three distinct operations. The astronomers of the continent of Europe, and particularly in France, where the art of dividing, except of kingdoms, is almost unknown, have been lavish of their praises of this instru ment; while in this country, where that art is practised with equal success upon minuter objects, a prejudice against it has too long continued. As a proof of this, we believe that the late Mr Ramsdell never made either a re flecting or a repeating circle. The prejudice is now, how ever, fist wearing off, and this circle begins to take that rank among our instruments to which its merit entitles it. This we think will appear on perusing the following de scriptiomand from the examples, as communicated by dif ferent observers.
Plate is a perspective representation of the repeating circle as made in London, and exhibits, for aught we know, as perfect a construction of that instru ment as has yet appeared. In our description of it we
shall, as we proceed, occasionally remark upon the dif ference between it and those made abroad, of which this professes to be in many respects a copy.
At bottom, a strong tripod with feet screws rests upon a pedestal. Two of the screws bear upon the concave tops of two plain brass studs, which are affixed to the pe destal, but the other screw rests upon au apparatus, whereby an exceeding slow motion is communicated to the instrument. This is represented at a. A steel spin dle 15 inches long, a full inch in diameter at one end, and somewhat more than half as much at the other, has its thicker end inserted into the middle of the tripod, to which it stands perpendicular. This spindle or axis is worked, especially near the ends, with the most scrupulous atten tion to truth. A strong hollow brass pillar is nicely cen tered upon the extreme ends of the spindle, round which it turns with a free and steady motion, and resting with its base upon the middle of the tripod, sustains the whole weight of the incumbent instrument.
The azimuth circle, 12 inches in diameter, united by 6 radii and a circular cent•e-piece to the pillar, moves round with it near to the upper surface of the tripod, but yet -,without touching it. This circle is graduated by lines into parts of It/ each, which are met, and subdivided to 16", by a vernier, as at b, attached to each branch of the tri pod. All this is executed with the greatest care, in or der that an angle may be well ascertained by a single ob servation, in cases where extreme accuracy is not wanted ; and as the business of repeating is a very laborious task, and productive of a vast waste time, this substitute will be considered by most observers as a very convenient re lief. It may here be noticed, that, as the position of the telescope is fully three inches from the vertical axis, all angles, except in astronomy, and where land objects are very remote, must be subjected to a consequent correc tion, in proportion to the distance of the objects from the station.