parts supported in such a manner as to resist the excess of motion they are capable of acquiring by virtue of their mass. This last circumstance is frequently overlooked by such workmen as are employed in the package of instruments; whence it necessarily follows, that some strain or fracture must be produced when matters of very unequal density are exposed to receive a common impulse.

To find the specific gravity of any solid by the gravimeter, observe this rule: "From the weight in the upper dish, when the instrument is properly immersed in the unknown fluid, take the weight which is placed with the body in the same scale at the like adjustment. The remainder is the absolute weight of the solid. Multiply this by the specific gravity of the fluid, and reserve the product. From the additional weight when the body is placed in the lower basin, take the weight when it was placed in the upper. The remainder will be the loss of weight by immersion. Divide the reserved product by the loss by immersion, and the quotient will be the specific gravity of the solid with regard to distilled water at the standard temperature and pressure."

To find the specific gravity of a fluid proceed thus: "To the weight of the gravimeter add the weight required in the upper basin to sink it in the unknown fluid. Again, to the weight of the gravimeter add the weight required in the same manner to sink it in distilled water. Divide the first sum by the latter, and the quotient will be the specific gravity of the fluid in question.'

For figures of the gravimeter, see Annales de Chimie, tome 21, or Nicholson's Journal, vol. I. 4to.

GUDGEONS, in machinery, having all the weight on the shaft to support, ought to be made sufficiently strong for that purpose; while, to avoid unnecessary friction, they should be made as small in diameter as possible, consistently with the requisite strength and durability.

Wrought iron being stronger than cast iron in about the ratio of 7 to 5, will bear a greater weight; yet, cast iron being cheaper, and more easily shaped, is more frequently employed for gudgeons.

Mr. Buchanan, who has paid considerable attention to this subject, gives these rules for the gudgeons of water-wheels.

1. The cube-root of the weight of a water-wheel in hundred weights, is nearly equal to the diameter in inches, of a cast-iron gudgeon sufficiently strong to support such wheel.

2. For wooden water-wheels, multiply the diameter in feet By the width also in feet, to which add the square of half the

diameter: the cube root of the sum will be nearly equal to the diameter of the gudgeon in inches.

These, of course, must be regarded as approximations.

Mr. Buchanan has inferred from several experiments, that gudgeons of the same size, of cast and of wrought iron, are capable, at a medium, of sustaining weights without flexure, in the proportion of 9 to 14."

Upon this principle Mr. B. computed the following table, to show the proportionate diameters of cast-iron and wroughtiron gudgeons.

Explanation of the table of cast-iron and wrought-iron gudgeons.

Column 1 and 2 are the same as those in the table of castiron gudgeons.

Column 3 contains numbers in the proportion of 9 to 14 less than those in column 2.

Column 4 contains the cube root of column 3, or the diameters of wrought-iron gudgeons, having the same strength as those of cast-iron in column 1.

USE OF THE TABLE.

Example.

To find the diameter of a wrought-iron gudgeon of the same strength with one of cast-iron of 3 inches diameter. Look on the 1st column for 3, and on the same line in the 4th column will be found 2-571282, that is, a little more than 2 inches, the diameter required of the wrought-iron gudgeon.

The numbers in the 3d column, being the cube of those in the 4th, another use may be made of this part of the table. For, supposing the 4th column to represent cast-iron gudgeons, the 3d column will represent the hundred weights which castiron gudgeons of those diameters should sustain.

cast-iron iron gudgeon, or the cwts. Cube of diameter of

4

gudgeons in which the gudgeons may

wrought-iron gudgeons.

inches. sustain.

Diameter of wrought-iron gudgeons in inches and parts.

:

: HANDMILLS, are commonly used for some culinary purposes, as the grinding of coffee, pepper, and the like. Sometimes handmills of larger size are used to grind malt, wheat, &c. and in such cases the hand is generally applied to a winch handle. But in Bockler's Theatrum Machinarum there is a description of a mill, in which the effort of a man is applied to a lever moving to and fro horizontally, nearly as in the action of rowing as this is a very advantageous method of applying human strength, the effort being greatly assisted by the heaviness of the man in leaning back, we shall give a brief description of this kind of mill, which is represented in fig. 4. pl. XII. The vertical shaft EG carries a toothed wheel c, and a solid wheel F; the latter being intended to operate as a regulating fly. Upon the crank AB hangs one end of an iron bar 1, the other end of which hangs upon the lever HK; the motion being pretty free at both ends of this bar 1. One end of the lever HK hangs upon the fixed hook K, about which as a centre of motion it turns. Then, while a man, by pulling at the lever нк, moves the extremity H from H to N, the bar 1 acting upon the crank AB gives to the wheels c and F half a rotation; and the momentum they have acquired will carry them on, the man at the lever suffering it to turn back from N to H, while the other half of the rotation of the wheels is completed. In like manner another sufficient pull at the lever HK gives another rotation to the wheel c, and so on, at pleasure. The wheel c turns by its teeth the trundle D, the spindle of which carries the upper mill-stone, just as the spindle D carries round the upper stone in fig. 1. pl. XV.

In this mill the nearer the end of the bar 1 upon the lever HK is to the fixed hook K, the easier, cæteris paribus, will the man work the mill. If the number of teeth in the wheel c be 6 times the number of cogs in the trundle D, then the labourer by making 10 pulls at the lever H in a minute will give 60 revolutions to the upper mill-stone in the same space of time.

The Society of Arts have lately adjudged a silver medal to Mr. Garnett Terry, of City Road, Finsbury-square, for his invention of a mill to grind hard substances, by means of a wheel turning upon a horizontal axis instead of a vertical one, as in the common construction. Mr. Terry has constructed this mill on a large scale; there is also a model deposited in the collection of that society.

Plate VIII. fig. 4. A. The hopper or receptacle of the articles which are intended to be ground.

B. A spiral wire, in the form of a reversed cone, to regulate the delivery of them.

c. An inclined iron plate, hung upon a pin on its higher end : the lower end rests on the grooved axis D, and agitates the wire B

D. The grooved axis, or grinding cylinder, which acts against the channelled iron plate E.

F. A screw on the side of the mill, by means of which the iron plate E is brought nearer to or removed further from the axis D, according as the article is wanted finer or coarser.

G. The handle by which motion is given to the axis.

H. The tube from whence the articles, when ground, are received.

*The front of the mill is taken off, in order to shew its interior construction.

HEART-WHEEL is the name given in England to a well known method of converting a circuitous motion into an alter nating rectilinear one, which is common in cotton-mills. It is an ellipse turned either on an axle, or by means of a winch and handle on one of its foci, or its centre, on whose edge a moveable point or circle presses; the latter receives an alternating motion from the circumference of the ellipse, and presses it in its revolution to different distances from the centre of motion. This method was contrived, we believe, by Sir Samuel Morland, about the year 1685. The practical disadvantages of this contrivance are the inequality of pressure and of moving force which will be required at different parts of the rotation of the ellipse, and the consequent wearing of some parts of it much faster than others, which will render it frequently necessary to have new elliptical wheels. A late application of the heart-wheel has been already mentioned, under the word COINING.

HOOKE'S JOINTS, or, as they are often called, universal joints, have been described in the introductory part of this volume.

HUNTER'S DOUBLE SCREW, was described in art. 161. vol. I.

HYDRAULIC MACHINES, are structures contrived for the purpose either of conveying water from one situation to another, particularly from a lower to a higher; or, by means of the force or pressure of water, to perform some mechanical operation, as grinding, boring, sawing. The former kind of hydraulic engines will only be spoken of here; the latter being described under the various heads FLOUR-MILL, FLAX-MILL, SAW-MILL, &c.

1. Of all the machines the ancients invented to raise water, it appears that though Archimedes's screw (see Archimedes's SCREW in this volume) was the most curious, the tympanum, mentioned by Vitruvius, elevated the greatest quantity at once: