in the sides of the bracket half an inch deep, and an eighth of
an inch wide, to receive the two wire pivots of a roller; which roller
is composed of a cylinder, three inches long and half an inch
diameter; and a cone three inches long and one inch diameter in its
largest part or base. The cylinder and cone are not separate, but are
turned out of one piece; a string is fastened to the cone at its base
_a_, with a bullet or any other small weight at the other end of it;
and another string and weight are fastened to the cylinder at _c_; the
pivot _p_ of wire is bent into the form of a handle; if the handle is
turned either way, the strings will be respectively wound up upon the
cone and cylinder; their lengths should now be adjusted, so that when
the string on the cone is wound up as far as the cone will permit, the
two weights may be at an equal distance from the bottom of the
bracket, which bottom we suppose to be parallel with the pivots; the
bracket should now be fastened against a wall, at such a height as to
let the weights lightly touch the floor when the strings are unwound:
silk or _bobbin_ is a proper kind of string for this purpose, as it is
woven or plaited, and therefore is not liable to twist. When the
strings are wound up to their greatest heights, if the handle be
suddenly let go, both the weights will begin to fall at the same
moment; but the weight 1, will descend at first but slowly, and will
pass through but small space compared with the weight 2. As they
descend further, No. 2 still continues to get before No. 1; but after
some time, No. 1 begins to overtake No. 2, and at last they come to
the ground together. If this machine is required to show exactly the
space that a falling body would describe in given times, the cone and
cylinder must have grooves cut spirally upon their circumference, to
direct the string with precision. To describe these spiral lines,
became a new subject of inquiry. The young mechanics were again eager
to exert their powers of invention; the eldest invented a machine upon
the same principle as that which is used by the best workmen for
cutting clock fusees; and it is described in Berthoud. The youngest
invented the engine delineated, Plate 1, Fig. 4.

The roller or cone (or both together) which it is required to cut
spirally, must be furnished with a handle, and a toothed wheel _w_,
which turns a smaller wheel or pinion _w_. This pinion carries with it
a screw _s_, which draw