are put in motion, and will be nearer the front of the carriage. This point, however, will vary with the weight of the load; but it is sufficiently obvious that the friction roller should be at a little distance from the lowest point of the axle

tree.

Mr. Gammett of Bristol has applied friction: rollers in a different manner, which does not, like the preceding method, weaken the axle-tree.Instead of fixing them in the iron part of the axle, he leaves a space between the nave and the axis to be filled with equal rollers almost touch-ing each other. The axis of these rollers are inserted in a circular ring at each end of the nave, and these rings, and consequently the rollers, are kept separate and parallel, by means of small bolts passing between the rollers from one side of the nave to the other.

In wheel-carriages constructed in the common manner with conical rims, there is a great degree of resistance occasioned by the friction of the linch pins on the external part of the nave, which the ingenious mechanic may easily remove by ajudicious application of the preceding principles.

As it appears from the experiments of Fer guson and Coulomb, that the least friction is generated when polished iron moves upon brass, the gudgeons and pivots of wheels, and the axles of friction rollers, should all be made of polished iron, and the bushes in which these gudgeons move, and the friction wheels should be formed of polished brass.‘

When every mechanical contrivance has been Friction adopted for diminishing the obstruction which diminished

M. de La Hire recommends the sockets or bushes to be made square and not concave.

by un

guents.

By the

power of

arises from the attrition of the communicating
parts, it may be still farther removed by the
judicious application of unguents.
The most
proper for this purpose are swines grease and
tallow, when the surfaces are made of wood,
and oil when they are of metal. When the
force with which the surfaces are pressed toge-
ther is very great, tallow will diminish the fric-
tion more than swines grease. When the wood-
en surfaces are very small, unguents will lessen
their friction a little, but it will be greatly di-
minished if wood moves upon metal greased with
tallow. If the velocities, however, are increased,
or the unguent not often enough renewed, in
both these cases, but particularly in the last, the
unguent will be more injurious than useful.
The best mode of applying it, is to cover the
rubbing surfaces with as thin a stratum as pos-
sible, for the friction will then be a constant
quantity, and will not be increased by an aug-
mentation of velocity.

In small works of wood, the interposition of black lead. the powder of black lead has been found very useful in relieving the motion. The ropes of pulleys should be rubbed with tallow, and whenever the screw is used, the square threads should be preferred.

MECHANICS.

ON THE NATURE AND OPERATION OF FLY WHEELS.

inequal mo

A FLY in mechanics is a heavy wheel or cy- Fly wheels. linder which moves rapidly upon its axis, and is applied to machines for the purpose of rendering uniform a desultory or reciprocating motion, arising either from the nature of the machinery, from an inequality in the resistance to be overcome, or from an irregular application of the impelling power. When the first mover is in Causes of animate, as wind, water, and steam, an inequa- tion in ma lity of force obviously arises from a variation chines. in the velocity of the wind, from an increase of water occasioned by sudden rains, or from an augmentation or diminution of the steam in the boiler, produced by a variation in the heat of the furnace; and accordingly various methods have been adopted for regulating the action of these variable powers. The same inequality of force obtains when machines are moved by horses or men. Every animal exerts its greatest strength when first set to work ter pulling for some time its strength will be impaired, and when the resistance is great, it will take frequent, though short relaxation, and then commence its labour with renovated vigour. Vol. II. Z

Af

remedied

by a fly.

These intervals of rest and vigorous exertion must always produce a variation in the velocity of the machine, which ought particularly to be avoided, as being detrimental to the communicating parts as well as the performance of the machine, and injurious to the animal which is These in- employed to drive it. But if a fly, consisting equalities either of cross bars, or a massy circular rim, be connected with the machinery, all these inconveniencies will be removed. As every fly wheel must revolve with great rapidity, the momentum of its circumference must be very considerable, and will consequently resist every attempt either to accelerate or retard its motion. When the machine, therefore, has been put in motion, the fly wheel will be whirling with an uniform celerity, and with a force capable of continuing that celerity when there is any relaxation in the impelling power. After a short rest the animal renews his efforts, but the machine is now moving with its former velocity, and these fresh efforts will have a tendency to increase the velocity: the fly, however, now acts as a resisting power, receives the greatest part of the superfluous motion, and causes the machinery to preserve its original celerity. In this way the fly secures to the engine an uniform motion, whether the animal takes occasional relaxation or exerts his force with redoubled ardour.

Exempli- We have already observed, that a desultory fied in a or variable motion frequently arises from an inmachine. equality in the resistance, or work to be perform

thrashing

ed. This is particularly manifest in thrashingmills, on a small scale, which are driven by water. When the corn is laid inequally on the feeding board, so that too much is taken in by the fluted rollers, this increase of resistance in

stantly affects the machinery, and communicates a desultory or irregular motion even to the water wheel or first mover. This variation in the velocity of the impelling power may be distinctly perceived by the ear in a calm evening, when the machine is at work. The best method of correcting these irregularities is to employ a fly wheel, which will regulate the motion of the machine, when the resistance is either augmented or diminished. In machines built upon a large scale there is no necessity for the interposition of a fly, as the inertia of the machinery supplies its place, and resists every change of motion that may be generated by an inequal admission of the corn.

ties arise

from the

the machi.

nery.

A variation in the velocity of engines arises Irregularialso from the nature of the machinery. Let us suppose that a weight of 1000 pounds is to be nature of raised from the bottom of a well 50 feet deep, by means of a bucket attached to an iron chain which winds round a barrel or cylinder; and that every foot in length of this chain weighs 2 pounds it is evident that the resistance to be overcome in the first moment is 1000 pounds, added to 50 pounds, the weight of the chain ; and that this resistance diminishes gradually, as the chain coils round the cylinder, till it becomes only 1000 pounds, when the chain is completely wound up. The resistance therefore decreases from 1050 to 1000 pounds; and, if the impelling power is inanimate, the velocity of the bucket will gradually increase; but if an animal is employed, it will generally proportion its action to the resisting load, and must therefore pull with a greater or less force, according as the bucket is near the bottom or top of the well. In this case, however, the assistance of a fly may be