A barometer of this construction has decided ad vantages with respect to the extent of hs changes, but mill it is not exempt from considerable defects. The moisture on the inner surface of the cylindrical re servoir increases the adhesion of the mercury, and retards its movements. But a much greater source of error proceeds from the influence of heat in ex tending the volume of liquor contained in that re servoir, and rising into the narrow stem. This in strument, therefore, to a certain extent, blends the indications of the barometer with those of the ther mometer, which are essentially different, and oaa sel dom accord.
• About the seine period, Dr Hook, likewise pro. posed a double barometer, of a similar construction. Be afterwards resumed the subject, and with a view to cermet the defect of the former arrangement he produced, in 1685, as instrument of a more complex hem, but eery ingeniously conceived. (See fig. 6.) To the upper end of the open stem, he joined a third cylinder of the same dimensions as the two former, bat tapering away to a fine orifice at the top. The piaci* tube being filled as usual with mercury, extending to occupy the bottoms of both the con nected cylinders, he introduced a liquor ineozedis ately over the mercury in the second cylinder, risegartly into the stem ; above this, again he penned another liquor specifically lighter and direr eatly coloured, filling up the rest of the stem and mounther into the third cylinder. By this artificial and delicate ocineination, the mercury is left per fectly stationary, mad all the monism* correspond ing to the changes of atmospheric pressure, are per tbrmed by the coanterpoising liquors, and marked by their hoe of mutual separation. Since the stem or •arrow tube manias constantly NI, the variation of he pressure shut depend on the different time of its length occupied by the two fluids. If the atmosphere should, for stance, the denser liquor will rise, and Cause the lighter liquor to contract its The action of this compound barometer, Wag thus produced merely by the difference of ties amity of the two fields, might, therefore, be augmented intiefinitely. Suppose the liquid testing
oh the mercury to be pure water, and the superin ourtbeat liquid to be olive oil, which is about one. Mehl& part lighter ; the *tale *meld be enlarged no less than one hundred and sixty-three times, or an alteration of one-tenth in the altitude of the com mon mercurial column, would be marked by a mo tion through 12 X 1.86 inches, or 16.8 inches. But such a vast enlargement of the scale is far greater than would ever be desirable in practice. it were better, therefore, to introduce next the mercury some fluid which is denser than water. If oil of sassafras were combined with oil of oranges, the divi sions of the scale would be augmented only sixty eight times, and consequently the whole range might not exceed ten or twelve feet. Those oils, however, would move rather sluggishly, especially in cold weather, and might, from their incessant shift info, during a lengthened period, become insensibly mixed. On the other hand, fluids of distinct cha racters are seldom free from chemical action ; they expand differently with heat, and by coating with other traces the inside of the tube, they are the more apt to retard the motion of the column. In general, the advantage of any very great augmentation of the scale is connteltalanced, as the fluids then work by irregular starts ; and the instrument loses in delicacy whatever it has gained in extent of action.
Another method of augmenting the variations of the barometer was invented by the same fertile genius, which has the advantage of uniting great simplicity with tolerable accuracy. (See fig. 7.) Resuming the syphon barometer, he made a small float of iron or glass to rest on the exterior surface of de mercury, and suspended by a slender thread passed round a small wheel or cylindrical axis that carried an index. Though the varieties of the height of the mercurial Oolumn are in a tube of this form, reduced to half the ordinary measure; yet, from the great leno.th of the index compared with the diameter of its axis, the divisions on the circumference of the circle in which it travels are much amplified. The little machine/ being concealed within the frame of the instrument,. the index only is brought into view, protected by a circular plate of glass. Thus fitted up, the whole forms rather a handsome piece of furniture. The Wheel Barometer, as it is called, has long maintained its reputation among ordinary observers.