1. Torricelli’s cistern barometer. 2. Torricelli’s syphon barometer. 3. Huygen’s barometer. 4. Huygen’s modified. 5. Wheel barometer. 6. Bernoulli’s syphon barometer. 7. Aminton’s conical barometer. 8. Gay Lussac’s barometer. 9. Gay Lussac’s barometer modified by M. Bunten. a, Tube containing a column of mercury. b, Mercurial cistern. c, A column of mercury supporting another of water, d. e, e, Weights, one of which floats on the surface of the mercury, and by means of the cord f moves the index g. h, Graduated dial. k, Capillary hole drilled laterally to admit air.

The construction of a barometer requires the utmost skill and care of a practised artist, and will therefore be seldom undertaken by the amateur or experimentalist—a fact which renders it unnecessary for us to enter into the details here. In the choice of his instrument the purchaser must greatly depend on the known experience and integrity of the manufacturer; as nothing but lengthened use, and frequent comparisons with other instruments, can possibly prove its excellence. An ordinary barometer, however carefully made, is found to suffer gradual deterioration, from the external air insinuating itself between the mercury and the glass tube, by which the perfection of the vacuum is destroyed. Various plans have been proposed to remedy this inconvenience and source of error. Prof. Daniell forms the bottom part of the tube, to the extent of about

1

⁄

3

rd of an inch, of solid platinum, welded to the glass. This plan has proved completely satisfactory. Dr Ure proposes the use of platinum-foil for the same purpose. Before purchasing an instrument it is as well to ascertain that this has been done. In those called ‘

STANDARD BAROMETERS

’ the scale is movable and adjustable by a delicate screw, so as to enable the observer to bring the lower point or zero (0) of the scale coincident with the surface of the mercury in the cistern. Exact contact is readily effected by making the point, and its image as seen by reflection from the surface of the mercury, to coincide. In this case the cistern is made of glass. Provided the ivory scale be connected with the zero-point with a strip of brass, correction as to temperature is very nearly effected by this simple adjustment. The

WHEEL-BAROMETER

is chiefly serviceable as a domestic or land weather-glass.