Mcneild&rsquos Long Range Barometer

A barometer designed by a gentleman named McNeild is on a directly opposite principle to the one just described. The tube is made to float on the mercury in the cistern. It is filled with mercury, inverted in the usual manner, then allowed to float, being held vertically by glass friction points or guides. By this contrivance, the ordinary range of the barometer is greatly increased. One inch rise or fall in the standard barometer may be represented by four or five inches in this instrume t, so that it shows small variations in atmospheric pressure very distinctly. As the mercury falls in the tube with a decrease of pressure, the surface of the mercury in the cistern rises, and the floating tube rises also, which causes an additional descent in the column, as shown by fixed graduations on the tube. With an increase of pressure, some mercury will leave the cistern and rise in the tube, while the tube itself will fall, and so cause an additional ascent of mercury. This barometer is identical in principle with King’s Barograph (see p. 34).

The construction of Howson’s and McNeild’s Barometers has been assigned to Messrs. Negretti and Zambra. These instruments are usually made for domestic purposes with a scale of from three to five, and for public use from five to eight times the scale of the ordinary standard. Their sensitiveness is consequently increased in an equal proportion, and they have the additional advantage of not being affected by differences of level in the cistern. However, these novelties have not been sufficiently tried to determine their practical value for strictly scientific purposes; but as weather-glasses, for showing minute changes, they are superior to the common barometer.

The Water-glass Barometer.—If a Florence flask, having a long neck, have a small quantity of water poured into it, and then be inverted and so supported that the open end dips into a vessel containing water, a small column of water will be confined in the neck of the bottle, the pressure of which, upon the surface of the exposed water, will be equal to the difference between the atmospheric pressure and the elasticity of the confined air in the body of the bottle. As the pressure of the atmosphere varies, this column will alter in height. But the elasticity of the confined air is also subject to variations, owing to changes of temperature. It follows, then, that the oscillations of the column are dependent on alterations of temperature and atmospheric pressure. Such an arrangement has been called “the Water-glass Barometer,” and bears about the same relative value to the mercurial barometer, as an exponent of weather changes, that a cat-gut hygrometer bears to a thermometric hygrometer, as an indicator of relative moisture.