Fig. 2.

360 to 360 again; and then blow out the lamp, and take away the bar.

This done, put on an iron bar F where the brass one was before, and then set the index to the 360th degree again. Light the lamp and put it under the iron bar, and let it remain just as many seconds as it did under the brass one; and then blow it out, and you will see how many degrees the index has moved in the circle; and by that means you will know in what proportion the expansion of iron is to the expansion of brass; which I find to be as 210 is to 360, or as seven is to twelve. By this method, the relative expansions of different metals may be found.

The bars ought to be exactly of equal size; and to have them so, they should be drawn, like wire, through a hole.

When the lamp is blown out, you will see the index turn backward: which shews that the metal contracts as it cools.

The inside of this pyrometer is constructed as follows.

In Fig. 2, Aa is the short bar which moves between rollers; and, on the side a it has fifteen teeth in an inch, which take into the leaves of a pinion B (twelve in number) on whose axis is the wheel C of 100 teeth, which take into the ten leaves of the pinion D, on whose axis is the wheel E of 100 teeth, which take into the ten leaves of the pinion F, on the top of whose axis is the index above mentioned.

Now, as the wheels C and E have 100 teeth each; and the pinions D and F have ten leaves each, it is plain, that if the wheel C turns once round, the pinion F and the index on its axis will turn 100 times round. But, as the first

pinion B has only twelve leaves, and the bar Aa that turns it has fifteen teeth in an inch, which is twelve and a fourth part more; one inch motion of the bar will cause the last pinion F to turn a hundred times round, and a fourth part of a hundred over and above, which is twentyfive. So that if Aa be pushed one inch, F will be turned 125 times round.

A silk thread b is tied to the axis of the pinion D, and wound several times round it; and the other end of the thread is tied to a piece of slender watch-spring G, which is fixed into the stud H. So that as the bar ƒ expands, and pushes the bar A a forward, the thread winds round the axle, and draws out the spring: and as the bar contracts, the spring pulls back the thread, and turns the work the contrary way, which pushes back the short bar Aa against the long bar f. This spring always keeps the teeth of the wheels in contact with the leaves of the pinions, and so prevents any shake in the teeth.

In Fig. 1, the eight divisions of the inner circle Fig. 1. are so many thousandth parts of an inch in the expansion or contraction of the bars; which is just one thousandth part of an inch for each division moved over by the index.

A water-mill, invented by Dr. Barker, that has neither wheel nor trundle.

water-mill.

This machine is represented by Fig. 1 of Plate Barker's III, in which is a pipe or channel that brings PLATE III, water to the upright tube B. The water runs Fig. 1, Sur. down the tube, and thence into the horizontal trunk C, and runs out through holes at d and e

near the ends of the trunk on the contrary sides thereof.

The upright spindle D is fixed in the bottom of the trunk, and screwed to it below by the nút g; and is fixed into the trunk by two cross bars at f: so that, if the tube B and trunk C be turned round, the spindle D will be turned also.

The top of the spindle goes square into the rynd of the upper mill-stone H, as in common mills; and, as the trunk, tube, and spindle, turn round, the mill-stone is turned round thereby. The lower, or quiescent, mill-stone is represented by I; and K is the floor on which it rests, and wherein is the hole L for letting the meal run through, and fall down into a trough, which may be about M. The hoop or case that goes round the mill-stone rests on the floor K, and supports the hopper, in the common way. The lower end of the spindle turns in a hole in the bridgetree GF, which supports the mill-stone, tube, spindle, and trunk. This tree is moveable on a pin at h, and its other end is supported by an iron rod N fixed into it, the top of the rod going through the fixed bracket 0, and having a screw nut o upon it, above the bracket. By turning this nut forward or backward, the mill-stone is raised or lowered at pleasure.

While the tube B is kept full of water from the pipe A, and the water continues to run out from the ends of the trunk; the upper millstone H, together with the trunk, tube, and spindle, turns round. But, if the holes in the trunk were stopped, no motion would ensue ; even though the tube and trunk were full of water. For,

If there were no hole in the trunk, the pressure

of the water would be equal against all parts of its sides within. But, when the water has free egress through the holes, its pressure there is entirely removed: and the pressure against the parts of the sides which are opposite to the holes, turns the machine.*

* See Appendix for farther information on the construc tion of Dr. Barker's mill.-ED.

G 2

dox.

HYDROSTATICS.

A machine for demonstrating that, on equal bottoms, the pressure of fluids is in proportion to their perpendicular heights, without any regard to their quantities.

Hydrostati- THIS is termed the Hydrostatical Paradox: cal Para and the machine for shewing it is represented in PLATE III, Fig. 2 of Plate III.--In which A is a box that Fig. 2, Sup. holds about a pound of water, abcde a glass

tube fixed in the top of the box, having a small wire within it; one end of the wire being hooked to the end F of the beam of a balance, and the other end of the wire fixed to a moveable bottom, on which the water lies, within the box; the bottom and wire being of equal weight with an empty scale (out of sight in the figure) hanging at the other end of the balance. If this scale be pulled down, the bottom will be drawn up within the box, and that motion will cause the water to rise in the glass-tube.

Put one pound weight into the scale, which will move the bottom a little, and cause the water to appear just in the lower end of the tube at a; which shews that the water presses with the force of one pound on the bottom; put another pound into the scale, and the water will rise from a to b in the tube, just twice as high above the bottom as it was when at a; and then, as its pressure on the bottom supports two pound weight in the scale, it is plain that the pressure on the bottom is then equal to two