_Dial (or Wheel) Barometer._--The disposition which should be given to this instrument is precisely the same as that of the _Dial Thermometer_, described in a preceding section. You make a small iron weight float on the inferior surface of the mercury, and fix to this weight a silk thread, which is stretched by a counterpoise, and rolls over a very moveable pulley. The axis of this pulley carries a needle, which turns backwards or forwards according as the column of mercury augments or diminishes. You arrange the whole in such a manner that the extreme variations of this column cannot make the needle describe more than one circumference; with this view you give the pulley a diameter of nearly an inch. The dial barometer being rather an object of luxury than an instrument of precision, you graduate it by inscribing the following words, at full length, on the scale. In pl. 4, fig. 16, for example, you write, At the point _a_ Tempest. ... _b_ Much rain. ... _c_ Rain or Wind. ... _d_ Temperate. ... _e_ Fine Weather. ... _f_ Fixed Fair. ... _g_ Very Dry. You write nothing at the inferior division. * * * * * GRADUATION OF THE MANOMETER.--The graduation of this instrument consists in dividing the tube where the air is to be compressed, into a given number of parts of equal capacity; but as, in general, such tubes are employed as are nearly capillary and very regular, the operation is reduced to a linear division, where every degree occupies an equal space. * * * * * GRADUATION OF THERMOMETERS. _Construction of Standard Thermometers._--Having constructed your instrument with a very regular tube, or one which has been divided into parts of equal capacity, and having filled it with the proper liquid, according to the instructions given in a preceding section, the graduation is to be effected as follows. Procure very pure ice, break it into small pieces, and fill a vessel with it. When the ice begins to melt, plunge the thermometer into the middle of it, in such a manner that, without touching the sides of the vessel, the whole thermometer, or at least that part of it which contains the liquid, may be covered with ice. Allow the instrument to remain in this state until, in spite of the gradual melting of the ice, the surface of the column of liquid remains at a fixed point, and neither falls nor rises. Mark this point very carefully on the stalk of the thermometer, either with a thread or a little drop of sealing-wax, or with the trace of a diamond or a flint. This is the _freezing point_, the _zero_ of the centigrade scale, the thirty-second degree of Fahrenheit’s scale.