TACHOMETER. A contrivance in vented for the purpose of indicating minute variations in the velocity of ma chines. When a vessel containing a fluid is whirled rapidly round a vertical axis, the centrifugal force produced by the whirling motion causes the fluid to re cede from the axis, and to rise on the sides of the vessel, so that the surface of the fluid assumes a concave parabolic form; and the distance to which the cen tre of the surface falls below its original level is proportionate to the velocity of rotation, and subject to corresponding variations. Any method, therefore, of measuring or rendering visible the de pression of the central surface will indi cate the variations of velocity. The method usually adopted is the following : A glass tube open at both ends, and ex panding at one extremity into a bell, is immersed with its wider end in mercury contained in a cup. The tube is so sus
pended as to be unconnected with the cup. This tube is then filled to a certain height, with spirits tinged with some coloring matter, to render it easily ob servable. The cup is attached to a spin dle turned by the machine. Now as the cup is whirled round by the spindle, the level of the mercury in the bell falls, and the spirit therefore descends in the tube. As the motion is continued, every change of velocity causes a corresponding change in the level of the mercury, and conse quently also in the height of the spirits in the tube • and as the capacity of the bell is much greater than that of the tube, a very small change in the level of the mercury causes a considerable change of the height of the spirits in the tube. TAFFETA. A light silk fabric, with a considerable lustre or gloss.