506 HYDROMECHANICS [HYDRAULICS. 1^7 Twin Floats Suppose two equal and similar floats (fig. 140) ] water, which ensures a constant coefficient of friction for the rubbing connected by a wire. Let one float be a little lighter and the other parts, and prevents any mud or grit finding its way m. In order a little heavier than water. Then the velocity of the combined floats will be the mean of the surface velocity and the velocity at the Fig. 139. Fig. 140. depth at which the heavier float swims, which is determined by the length of the connecting wire. Thus if v, is the surface velocity and Vd the velocity at the depth to which the lower float is sunk, the velocity of the combined iloats will be Consequently, if v is observed, and r s determined by an exp sriment with a single float, diam,. Fig. 141. According to Captain Cunningham, the twin float gives better i ^sults than the sub-surface float. 128. Velocity Rods. Another form of float is shown in fig. 141. This consists of a cylindrical rod loaded at the lower end so i$ to float nearly vertical in water. A wooden rod, with a metal cap at the bottom in which shot can be placed, answers better than anything else, and sometimes the wooden rod is made in lengths which can be screwed to- ether so as to suit streams of different epths. A tuft of cotton wool at the top serves to make the float more easily visible. Such a rod, so adjusted in length that it sinks nearly to the bed of the stream, gives directly the mean velocity of the whole vertical section in which it floats. 129. Revy s Current Meter. No instrument has been so much used in directly determining the velocity of a stream at a given point as the screw current meter. Of this there are a dozen varieties at least. As an example of the instrument in its simplest form, Mr Revy s meter may be selected. This is an ordinary screw meter of a larger size than usual, more carefully made, and with its details carefully studied (figs. 142, 143). It was designed after experience in gauging the great South American rivers. The screw, which is actuated by the water, is 6 inches in diameter, and is of the type of the Griffiths screw used in ships. The hollow spherical boss serves to make the weight of the screw sensibly equal to its dis placement, so that friction is much reduced. On the axis act of the screw is a worm which drives the counter. This consists of two worm wheels g and h fixed on a common axis. The worm wheels are carried on a frame attached to the pin I. By means of a string attached to I they can be pulled into gear with the worm, or dropped out of gear and stopped at any instant. A nut m can be screwed up, if necessary, to keep the counter per manently in gear. The worm is two-threaded, and the worm wheel g has 200 teeth. Consequently it makes one rotation for 100 rotations of the screw, and the number of rotations up to 100 is marked by the passage of the graduations on its edge in front of a fixed index. The second worm wheel has 196 teeth, and its edge is divided into 49 divisions. Hence it falls behind the first wheel one division for a complete rotation of the latter. The number of hundreds of rotations of the screw are therefore shown by the number of divisions on h passed over by an index fixed to g. One difficulty in the use of the ordinary screw m^ter is that particles of grit, getting into the working parts, very sensibly alter the friction, and therefore the speed of the meter. Mr Revy obviates this by enclosing the counter in a brass box with a glass face. This box is filled with pure Fig. 142. -Scale J full size. that the meter may place itself with the axis parallel to the current, it is pivoted on a vertical axis and directed by a large vane shown in fig. 1 43. To give the vane more direct ing power the vertical axis is nearer the screw than in ordinary meters, and the vane is larger. A second horizontal vane is attached by the screws x,x, the object of which is to allow the meter to rest on the ground without the motion of the screw being interfered with. The string or wire for starting and stopping the meter is carried through the centre of the vertical axis, so that the strain on it may not tend to pull the meter oblique to the current. The pitch of the screw is about 9 inches. The screws at x serve for filling the meter with water. The whole apparatus is fixed to a rod (fig. 143), of a length proportionate to the depth, or for very great depths it is fixed to a weighted bar lowered by ropes, a plan invented by Mr Revy. The instrument is generally used thus. The reading of the counter is noted, and it is put out of gear. The meter is then lowered into the water to the required position from a platform between two boats, or better from a temporary bridge. Then the counter is put into gear for one, two, or five minutes. Lastly, the instrument is raised and the counter again read. The velocity is deduced from the number of rotations in unit time by the formulae given below. For surface velocities the counter may be kept per manently in gear, the screw being started and stopped by hand. 130. The Harlachcr Current Meter. h. very interesting modification of the current meter is that made by Amsler Laflon of Schaffhausen, which is described in Hen- singer von Waldegg (Handb. dcr Ingcnicur- ^ , wisscnschaften, iii. p. 284). In this the or dinary counting apparatus is abandoned. A worm drives a worm wheel, which makes an electrical contact once for each 100 rotations of the worm. This contact gives a signal above water. With this arrangement, a series of velocity observations can be made, without removing the instrument from the water, and a number of practical difficulties attending the accurate starting and stopping of the ordinary counter are entirely got rid of. Fig 144 shows the meter. The worm wheel z makes one rotation for 100 of the screw. A pin moving the lever x makes the electrical contact. The wires b, c are led through a gas pipe B ; this also serves to adjust the meter
to any required position on the wooden rod dd. The rudder or
