384 Hydrometrical Observations. 
a tachometer or stream guage made by Mr Robinson, optician, London, 
and is drawn to a scale of one-third of the full size. In this view, ff 
represents what may be termed the driving vane, which is acted on by 
the stream, and of which g isa plan. The plane of this vane is twisted 
as represented by the dark shading in the cut, so as to present, not a 
knife-edge, but an oblique face to the action of the current, which, by 
impinging on it, causes it to revolve exactly in the same way that the 
wind propels the sails of a windmill. On the spindle or shaft of this 
vane, an endless screw is fixed at e, which works in the teeth of the first 
registering wheel, and causes it to revolve, when the vane is in motion 
and the screw in gear. Letters a and b represent a bar of brass, to which 
the pivots on which the registering wheels revolve, are attached. This 
bar is moveable on a joint at b; and at the point a, a cord ac is fixed, 
by pulling which the bar and wheels can be raised, and on releasing it 
they are again depressed by a spring at d. When the bar is raised, the 
teeth of the wheel are taken out of gear with the endless screw, and the 
vane is then left at liberty to revolve, the number of its revolutions being 
unregistered ; but when the cord is released, the spring forces down the 
wheels, and immediately puts the registering train into gear, in which 
state it is represented in the cut. Letter A is a stationary vane (which is 
shewn broken off, but measures about 9 inches in length) for keeping the 
plane in which the driving vane revolves at right angles to the direction 
of the current, and & is the end of a wooden rod to which the tachometer 
is attached when used. The different parts of the instrument itself are 
made of brass. 
The moveable bar for the registering wheels and the application of the 
cord and spring which have been described, afford the means of obsery- 
ing with great accuracy in the following manner. The instrument hav- 
ing been adjusted by setting the registering wheels at zero, or noting in 
the field book the figure at which they stand, the cord is pulled tight so 
as to raise them out of gear, and the instrument is then immersed in the 
water. The vane immediately begins to revolve from the action of the 
current, and is permitted to move freely round until it has attained the 
full velocity due to the stream. When this is supposed to be the case, 
a signal is given by the person who observes the time, and the register- 
ing wheels are at that moment thrown into gear by letting the cord slip. 
At the end of a minute another signal is given, when the cord is again 
drawn and the wheels taken out of gear, and on raising the instrument 
from the water, the number of revolutions in the elapsed time is read off- 
This operation being completed in the centre of each division of the cord, 
the number of revolutions due to the velocity at each part of the very 
line where the cross section is taken, is at once obtained. 
Before using the tachometer, it is obvious that the value of a revolution 
of the vane must be ascertained ; and although this is done by the manu- 
facturers, it is proper that the scale of each instrument should be deter- 
mined by the person who uses it, and that it be tested if the instrument 
