(a) Fig. 353 is a simple anemometer that anyone can make: a is the pressure-plate, exactly 6 in. square, made of galvanised iron and fastened to the pillar of a 10 lb. spring balance b; the cylinder of the balance being fixed to the vertical tube c, which carries the vane, etc, to the end of the iron rod, in balance, is attached a wire which passes over a wheel inside c, shown in drawing, but, of course, in the tube, and so that the copper wire can go down centre of c to the weight of df, which must have a slit in one side to run over a wire soldered inside f; this keeps lower wire straight and prevents torsion. The wire is joined up in two at the bottom of d, and the two wires should be continued down to the bottom. The vane is of the usual form, but should form a balance for the other side, and must be weighted to form the balance necessary. The wire is continued down into the room in which dial is Axed. Here make a dial 12 in. diameter and divide it into 36 divisions, each division being a full inch from the next: these divisions will indicate pounds, and if you divide the spaces between each into 5 you will have everything you require.
The centre wheel has a groove, and its circumference should be exactly the length of the rod in balance, or rather the length from 1 to 9 lb., every pound giving 4 lb. on dial. Now fix at the end a thin wire or watch-chain, the latter would be better, the old chains used in our grandfathers' verge watches - this, by the bye, must be connected to upper as well as lower wires; to the bottom one then hang a weight just sufficient to keep finger in place. Having all done, get another similar balance and fix it up against dial-plate, letting it mark 1 lb.; this will be 4 lb. on dial down below; mark this on the dial; then let upper balance be pushed on to 2 lb., mark 8 below, and so on till the whole dial is marked, then divide and mark pounds. You may go over this again and again until you have carefully marked the dial, and your anemometer will be finished. (W. J. L.)
(6) A multiplying anemometer, applicable to the measurement of the velocity of air-currents, to meteorological observations, and to the determination of waterflow, consists of a tube formed of two truncated conical tubes, the smaller ends of which are of the same area (Venturi's tubes). In this tube a much smaller one of similar construction is placed, as shown in Fig. 354. If greater delicacy be required, a third may be added, the whole system being eccentric. (Fig. 355). The constricted part of the outer compound tube is surrounded by a hollow jacket, and connected with it by the small interval which separates the two truncated cones. This jacket is in connection with a water-thermometer, which indicates the velocity of the current to be measured. This arrangement for a single compound tube is shown in Fig. 356. The utility of the instrument depends upon the fact that in such a case, as shown in Fig. 356, the reading of the manometer attached to the jacket is several times that indicated by a manometer at the orifice of the tube. The former is of course negative, whilet the latter is positive.
The relation between the two may be, for example, 6:1. In an instrument consisting of two compound tubes, and in one of three tubes, the readings were related to those at the orifice in the proportions 20: 1 and 80:1 respectively. The instrument is simple, rigid, portable, and inexpensive; it affords a check on the ventilating apparatus of mines, and by a simple clockwork arrangement could be made to indicate defective ventilation; lastly, its multiplied reading conduces to great accuracy.