MANOMETER

, or Manoscope, an instrument to shew or measure the alterations in the rarity or density of the air.

The Manometer differs from the barometer in this, That the latter only serves to measure the weight of the atmosphere, or of the column of air over it; but the former, the density of the air in which it is found; which density depends not only on the weight of the atmosphere, but also on the action of heat and cold, &c. Authors however often confound the two together; and Mr. Boyle himself has given a very good Manometer of his contrivance, under the name of a Statical Barometer, consisting of a bubble of thin glass, about the size of an orange, which being counterpoised when the air was in a mean state of density, by means of a nice pair of scales, sunk when the atmosphere became lighter, and rose as it grew heavier.

The Manometer used by captain Phipps, in his voyage towards the North Pole, consisted of a tube of a small bore, with a ball at the end. The barometer being at 29.7, a small quantity of quicksilver was put into the tube, to take off the communication between the external air, and that confined in the ball and the part of the tube below this quicksilver. A scale is placed on the side of the tube, which marks the degrees of dilatation arising from the increase of heat in this state of the weight of the air, and has the same graduation as that of Fahrenheit's thermometer, the point of freezing being marked 32. In this state therefore it will shew the degrees of heat in the same manner as a thermometer. But when the air becomes lighter, the bubble inclosed in the ball, being less compressed, will dilate itself, and occupy a space as much larger as the compressing force is less; therefore the changes arising from the increase of heat, will be proportionably larger; and the instrument will shew the differences in the density of the air, arising from the changes in its weight and heat. Mr. Ramsden found, that a heat equal to that of boiling water, increased the magnitude of the air, from what it was at the freezing point, by 414/1000 of the whole. Hence it follows, that the ball and the part of the tube below the beginning of the scale, is of a magnitude equal to almost 414 degrees of the scale. If the height of both the Manometer and thermometer be given, the height of the barometer may be thence deduced, by this rule; as the height of the Manometer increased by 414, to the height of the thermometer increased by 414, so is 29.7, to the height of the barometer; or if m denote the height of the Manometer, and t the height of the thermometer; then , which is the height of the barometer.

Another kind of Manometer was made use of by colonel Roy, in his attempts to correct the errors of the barometer; which is described in the Philos. Trans. vol. 67, pa. 689.