THE EOYAL AETILLEEY INSTITUTION. 
171 
E is connected with the screens, and records the passage of the shot through 
the screens. By comparing the marks made by m, ml the exact velocity of 
the shot can be calculated at all points of its course. The slide E is fixed 
parallel to F and the cylinder K by the brackets G, H. Y is a screw for 
drawing back the wheelwork M 3 and J a stop to regulate the distance 
between M and B. The depression of the lever h raises the two springs s, 
which act as levers, and bring the diamond points m, ml down upon the 
paper. When an experiment is to be made, care is taken to see that the 
two currents are complete. The fly-wheel A is set in motion by hand, so as 
to make about three revolutions in two seconds. The markers m, ml are 
brought down upon the paper, and after four or five beats of the clock the 
signal to fire is given, so that in about ten seconds the experiment is com¬ 
pleted, and the instrument is ready for another. 
Fig. 2 gives a full-sized view of one of the markers, shewing the way in 
which it is moved. The depression of the lever li (Fig. 1), raises jo, and 
thus the lever s, which is formed of watch-spring wire, brings down ml to 
the paper, and keeps it gently in contact. This motion takes place within 
the circle k, about an axis CD. a' is an arm connected with the electro¬ 
magnet. When the magnetism in E' is destroyed, a' begins to move away, 
and when it has moved a short distance it strikes the lever b' a sudden blow 
which carries it as far as the hole in the stop c' will allow it to move. The 
The lever V is rigidly connected with the circle k 3 which is capable of moving 
about an axis AB. This motion is communicated to m' } which describes a 
very short arc of a circle about a point in AB. The arrangement is so 
made that when either of the markers m , ml is making a record, it has a 
motion which may be resolved partly in direction of the motion of the paper 
under it, and partly in a direction perpendicular to this. Thus records are 
obtained which can be read off by scale with great nicety. 
The pendulum of a half-seconds clock strikes once each double-beat a 
very light spring, and so interrupts the galvanic current in E' once a second. 
