179 MISCELLANEOUS INTELLIGENCE. I. MECHANICAL SCIENCE. 1. Ascent of Hot Air through Tubes.-A very extensive series of experiments has been made on the velocity with which hot air ascends through tubes, the object being to ascertain the principal points which are necessary to be considered in the construction of chimneys: for this reason, the experiments were made as much as possible to resemble the state of things in actual flues, and the results are in consequence directly applicable, and of an increased value. The account of the whole is published, in a Traité de la Chaleur, 2 vols. 8vo., and also in the Annales de l'Industrie, ii. p. 110. The conclusions, for which only we have room, are as follow:- i. The tunnels or flues oppose a resistance to the motion of hot air through them, in the direct proportion of the length of the tunnel and the square of the velocity, and inversely as the diameter. ii. The co-efficient of friction is not the same for different substances. iii. On contracting the upper orifice, the velocity of the air at the orifice increases to a certain limit, which is the velocity due to the pressure existing in the lower part of the tunnel. iv. On contracting the lower orifice, the quantity passed diminishes only in the proportion of the diameter of the orifice, and, consequently, the velocity in the orifice itself increases in the inverse ratio of its diameter. The two last results will have numerous applications in the arts. A powerful draught is often indispensable, and is always useful in economizing the fuel. Up to this time, only two elements of draught have been considered,-the height of the chimney, and the temperature of the hot air; but the height is increased only at a serious expense, and to increase the temperature of the air requires the consumption of fuel. From the facts stated in the memoir, it appears that the diameter of the chimney is also an important element of draught; limited when the upper orifice is constant, but indefinite when that is free, and costing but little in construction. Thus, by augmenting the diameter of chimneys, and leaving the upper orifice free, or of a constant diameter, an increased draught may be obtained even when the temperature of the ascending column is diminished. 2. On the Construction of Vitro-Crystalline Object Glasses.Achromatic object glasses for telescopes are constructed usually of two or more lenses, some of which are of flint glass, the others of crown glass, the achromatic effect depending upon the arrangement of two different media, possessing different dispersive powers. The scientific world is well acquainted with the endeavours which have been made, for years past, to improve the heavier of these two glasses, namely, the flint glass. In the mean time, M. Cauchoix, of |