BAROMETER, (Encycl.) A considerable improvement on the wheel-barometer † is that of Mr Fitzgerald, F.R.S. It is furnished with two pulleys See Barometer, No 24. that move on friction-wheels; each of which turns an index on the centre of a graduated circle. The smaller circle is four inches in diameter, and divided into three equal parts, each of which is again subdivided decimally; and the changes corresponding to the rise or fall of the mercury from 28 to 31 inches, are marked on the margin of it, as they are on the scales of common barometers. The larger circle is divided into 300 equal parts; and being about 30 inches in circumference, the index belonging to it will mark distinctly the 600th part of an inch in the rise or fall of the mercury. On the centre of this circle two registers are fixed, which are placed along the index when the instrument is adjusted; one of them is carried round with the index, and left behind on its return; so that their distance will determine the limits of the variation from one observation to another.
Mr Passemont, an ingenious artist at Paris, has invented a marine barometer, by twisting the middle part of the common one into a spiral consisting of two re-
volutions. Thus the impulses which the mercury receives from the motions of the ship are destroyed, by being transmitted in contrary directions.
Notwithstanding the amazing pains which Mr de Luc has taken to remove every inaccuracy in the barometer, it is not yet entirely free from error; nor do the observations made by different persons altogether correspond. Considerable improvements have been made by Col. Roy and Sir George Shuckburgh, of which an account is given in the 67th and 68th vols of the Philosophical Transactions.