Manometer an instrument for measuring the rarefaction and condensation of elastic fluids, but especially that of the atmosphere. It differs from the barometer which shows only the weight of the superincumbent column of air; whereas the manometer shows the density, which depends on the combined effect of weight and the action of heat. It is sometimes called manoscope. Among the various contrivances of this kind may be mentioned that of the Hon. Robert Boyle, which he calls a statical barometer; which consists of a bubble of thin glass about the size of an orange, which being counterpoised in an accurate pair of scales, rises and sinks with the alterations of the atmosphere. This instrument, however, does not show the cause of the difference of density in the atmosphere, whether it be from a change of its own weight, or its temperature, or both. The manometer constructed by Mr. Ramsden, and used by Capt. Phipps, in his voyage to the North Pole, was composed of a tube of small bore with a ball at the end; the barometer being 2.97, 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 if the air becomes lighter, the bubble inclosed in the ball being less compressed, will dilate itself, and take up 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 show 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 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 determined also.