The wooden one of olden date, 9½in. long, 5½in. broad, 5½in. thick, weighs 5⅓lb., cubic measure 287⅓in.
Improved folding roof, &c., all iron, 8in. long, 4½in. broad, 2½in. thick, weighs 6¾lb., cubic measure 90in.
The improvements herein specified are not only its reduced size and weight, but its mechanical arrangements, form, and moderate price.
It consists of two circular disc-like reservoirs, about 2½in. in diameter, and ¾in. in depth, made of iron, at the same casting: one contains the mercury, and the other is the trough, fitted with glass cover for observing.
The discs are connected at their circumference by a narrow neck, and in it is drilled a hole, through which the mercury passes from one reservoir to the other; and this communication is opened or shut off by a stop-cock, on the cone principle, such as is used for water or gas, so that the mercury can be passed from one disc to the other without removing the glass cover, or the risk of losing any mercury.
The mercurial disc, A, is fitted with a cylinder-stopper, D, acting on a spiral spring, by which air can be admitted or allowed to escape.
The trough disc, B, is fitted with two glasses, G and E, which are ground mathematically parallel: one of the glasses is fitted to a frame, and screws on the disc, and is used while passing the mercury in or out of the trough; after which operation it is removed and replaced by the other glass, the edge of which next the stop-cock should be supported by the blade of a pocket-knife and then lowered on the mercury at the opposite side, and by a gentle pressure, force out the intervening air, leaving the glass to float on the surface of the mercury. Without this care, some of the mercury might be pressed over the edge of the disc.
The glass then presents a clear reflecting surface, which is not only protected from the effects of the wind, &c., but also maintains so great a steadiness as to mark a decided improvement over the old triangular glass roof which is placed over the mercury, instead of, as in this case, being on it.
It may be used afloat under favourable circumstances (the observer and artificial horizon being placed on a pendulum table). Another great advantage to this improved artificial horizon is the facility with which altitudes can be observed at 2° elevation, and consequently its adaptation for the measurement of very low stars, as well as the peaks of mountain ranges.
To return the mercury to its reservoir, remove the glass G that floats on the mercury, by lifting it up with the point of a knife, and then screw on the other glass cover, E. It is now only necessary to hold the instrument vertically, the trough end being uppermost (Fig. 4), turn the stop-cock, and press gently downward on the cylindrical stopper, and the mercury will rapidly return to its reservoir.