Description of a Reflecting Level or an Artificial Horizon, for taking Altitudes of the Celestial Bodies, &c. on Land by Hadley's Quadrant; with some Remarks on different Levels. By the Reverend JAMES LITTLE, With a Plate. From the TRANSACTIONS of the ROYAL IRISH ACADEMY, THE usefulness at sea of that excellent instrument, the Hadley's quadrant, is known to every navigator, it being the only one by which, without any stand or fixed position, and even when the observer is in motion, accurate observations can be made of the altitudes of the celestial bodies and their angular distances in order to find the longitude; and it would also, as being the most accurate and ready of all small and portable instruments of that nature, be exceedingly useful at land, especially to a traveller, in taking cursory observations, to discover the latitude, &c. of places occasionally visited, if it were furnished with an artificial horizon or level to supply the place of that natural one which at sea the water around the observer affords, and from which the altitudes of the celestial bodies are measured: for want of such a level on land, they who make observations with an Hadley's quadrant, which is not fixed and adjusted by a plummet or spirit-level, are obliged to take these altitudes by measuring the angular distance between the object and the image of the same seen on the smooth and level surface of a stagnant fluid, as water, quicksilver, &c.; and when the object is very remote, that angular distance will be double its elevation above the real horizon. But as such observations must be made in the open air, and the least breath of wind will ruffle the surface of the fluid, it is necessary to cover it with two glass planes, joined at their upper edges, and erected like the roof of a house over the vessel containing the fluid; each of these planes must have its opposite surfaces exactly parallel, which is never the case with plates of looking-glass, and the proper planes with the requisite apparatus are in few places to be procured, and liable to accidental injury. Instead of this fluid artificial horizon I have used the following one, which can any where be easily made, and which I found to answer well, as being accurate and portable, less liable to accidents in travelling than the former, and discovering in the act of observation its own errors of adjustment. I constructed it in the two forms hereafter described, the first of which is represented in a perspective view in Fig. 1, (Plate X), and in a section through the middle in Fig. 2: the second form appears in Fig. 8, in a like section through the common axis of its parts. It consists of a circular plate of glass A, (made of a piece of looking-glass) whose upper surface is plane and polished, but the under surface has its polish taken away by being rubbed with a piece of lead and flour of emery with water, so that an image of any object can appear by reflection from its upper surface only: to this is cemented, with black sealing-wax, the frame B C, (made of steel, or it might be of brass,) both having been previously made very clean and bright, heated in an oven, and then cooled slowly this frame is composed of the round plates B and C fastened together by the two rods or pillars a, a, strongly rivetted or brazed to both plates : the piece DEF is composed of the round plate D, which is exactly fitted and screwed close and immoveable by two screws to the plate C, so that both may be parallel to A and B. In the middle of the plate D is brazed the pipe : pipe pp: this pipe is made of steel, and its sides as thin and light as may be consistent with strength to secure it from being easily bent; it tapers a little towards its end, and its mean external diameter is about 3 of an inch: in the middle of this pipe is a steel-wire hardened towards the lower end, and tapering upwards towards E, where it is screwed (loosely) through the middle of a short pin or plate fastened in the mouth of the pipe, which pipe is (for strength) inserted quite through the plate D. The lower end of the wire, which extends a little below the pipe, has a hole through it at G at right angles to its axis, as represented (enlarged) in Fig. 4, and also in Fig. 5, in another section of the wire which is at right angles to the section in Fig. 4: this hole is to contain the knot of a looped thread t, which is inserted through â very small perforation made in the axis of the wire, and only so wide as just to admit the thread tight through it, or instead thereof a very fine and flexible string of cat-gut or fiddle-string, in order that when a weight is appended to the string it may pull the wire only by a point in its axis. A ring or hoop rr (of iron) is brazed on the lower end of the pipe pp, just so broad as to admit four equidistant and opposite adjusting screws ss (made of tempered steel, and having square heads, in order to be turned by a key as well as by a turn-screw) to be inserted' through it; and the hoop is to be so thick as to sustain, without change of its circular shape, the pressure of the screws against the wire GE. The pipe pp is to be fixed perpendicular in the plate D, that it, and also the wire EG, when in the middle of it, may be at right angles to the mirror; and then any little deviation of the wire from this will be corrected by moving the wire (which must be so fixed as to yield easily) towards that side where it makes too great an angle with the mirror, which is done VOL. II.-SECOND SERIES. Bb by |