Your most obedient humble Servant,
Kea. Fitz-Gerald.
Poland-Street, June 7th, 1758.
_Explanation of the Three Tables._
The wheel A (_Tab. 26._), which is turned by the lever B (_Tab. 27._),
or rather moved up and down by it, is loose on its arbor; and likewise
one of the rochets C (_Tab. 26._), and the wheel next to it D. The
outside rochet E, and outside wheel F, are fixed on the arbor.
There are two pinion-wheels G and H fixed on one arbor; one on each
side, near the edge of the wheel A, moved by the lever.
There are also two clicks _a_ and _b_; one _a_ fixed to the great wheel
A, the other _b_ fixed to the frame. These exclusive of the wheel I,
that moves the pinion _c_, on the arbor of which, the crank _d_, and
fly _e_, (_Tab. 27._) are fixed.
The effect is, when the lever B moves the wheel A downwards; its click
_a_, forces the rochet E, fixed on the arbor K, to move along with it,
and the other wheels the same way. When it moves upwards, the click _b_
fixed to the frame, stops the larger rochet C, and the wheel D next to
it, which are pinned together; and as the wheel A is carried upwards
by the lever, the pinion G towards the edge of it, is forced round the
wheel D, and moves the pinion H, on the other side the great wheel A,
which moves the wheel F fixed on the arbor K, the contrary way to the
wheel A. By which means, the arbor K is constantly turned the same way,
when the lever of the fire-engine moves either upwards, or downwards.
The pinion G, by being made proportionally smaller than the pinion H,
keeps the arbor K in the same swiftness of motion, when the lever is
moved upwards, as downwards.
[Illustration: _Philos. Trans. Vol. L._ TAB. XXVI. _p. 730_.
_J. Mynde sc._]
[Illustration: _Philos. Trans. Vol. L._ TAB. XXVII. _p. 730_.
_J. Mynde sc._]
[Illustration: _Philos. Trans. Vol. L._ TAB. XXVIII. _p. 730_.
_J. Mynde sc._]
The great wheel I, fixed on the arbor K, turns the pinion _c_, on the
arbor of which the crank _d_ (to move the ventilator), and the fly
_e_ (to help the motion), are fixed. The pinion _c_, is turned three
times by each alternate motion of the lever; which may be increased, or
diminished, according to the number of teeth in the pinion _c_.
The number of teeth in the wheel A is sixty-six, but need not be
toothed above half way. Instead of this wheel there might be a barrel,
with a chord round it, fixed at each end of the arch of the lever, and
projecting somewhat from it; which, by the motion of the lever, would
work in the same manner in other respects, and be easier made, and at
less expence.
The wheel D fixed to the rochet C has thirty-three teeth, and its
pinion G eleven.
The wheel F fixed on the arbor K has twenty-four teeth, and its pinion
H sixteen.
The greater the number of teeth in the rochets, the better.
_Tab. 26._ contains the plan (in parts) of the whole machine, except
the lever B and fly _e_, which are in _Tab. 27._
_Tab. 27._ also contains the elevation of the arbor, with its different
fixtures; _viz._
_Fig. 1._ The rochet C and wheel D (_Tab. 26._) fixed together.