1815.] the Rise and Fall of the Tide. 275 
of each of these wheels will pass by their respective arms or levers 
of the hammer in the same period, viz. 10 minutes: they therefore 
produce a common effect of lifting the hammer and letting it fall at 
every 10 minutes. The reason for employing two motions for this 
purpose is, that, if it was entrusted to the third wheel-to raise it 
with one pin, it might retard the motion of the clock, because that 
wheel has so slight a power; and, on the other hand, the 12 pins in 
the centre wheel would not be equally certain to drop the hammer 
exactly at the 10 minutes, because as it moves slowly any small in-~ 
equality in the arrangement of the pins would make a considerable 
difference in the time when the hammer was let fall. The pins of 
the centre wheel are therefore made to act first, and the wheel has 
suflicient power to lift the hammer without injury to the motion of 
the clock ; but just before this pin would let the hammer fall, the 
pin in the third wheel takes its lever and raises it up a very little 
higher, or rather holds it up at the same elevation, till the pin of 
the centre wheel has passed, and then at the expiration of the 10 
minutes it lets the hammer fall. 
The mark is made on the paper of the cylinder by a small piece 
of black lead penci!, which is fastened in a tube at the end of the 
hammer by a clamp screw: p is a small sliding weight upon the 
arm of the hammer, which can be fixed at any distance from the 
centre by a clamp screw, and will thus make the pencil strike with 
more or less force, as is found by experience to be necessary to make 
a clear and defined mark: one of the levers or arms of the arbor of 
the hammer must be made to fall upon a spring to stop the descent 
of the hammer. ‘This will yield sufficiently to allow the pencil to 
mark when falling with the blow, but will afterwards keep up the 
point so that it will not streak the paper. In this manner the pencil 
will make a row of dots round the cylinder as the tide rises, and the 
same as it falls: but to prevent the two rows falling upon the same 
line, by which they would confuse each other, a traversing motion 
is given to the cylinder at the same time that it turns round: this is 
effected by a worm P fixed upon the axis of the wheel, and a cock 
7 projecting from the standard D to carry a fixed pin which acts 
against the worm. By this means the row of dots, when the tide 
rises, is marked diagonally; but when the tide falls, the cock r 
quits the spiral, and the row of dots are marked circularly. The 
axis of the wheel is made with long pivots, which slide endways, to 
allow the side motion ; but the friction is sufficient to keep one side 
of the spiral worm in contact with the pin which acts against it 
whiist the wheel turns in one direction, and quits it when it turns 
in the contrary direction. 
As the paper must be changed aboutevery 13 hours, the bearing 
at the top of the standard C is made to open on a joint, that the 
wheel may be taken out; the sheet of paper is only confined by a 
or wire slipped over it; it is therefore easily changed ; and the 
only care is to make the end of the slip of paper to correspond with 
a line drawn upon the cylinder to represent the point from which 
the measurement is taken, or point of commencement. The tube 
s 2 
