In mortising machines with a reciprocating motion the base of the main column should be of massive construction, and the bed plate extended, as the duty in heavy machines is very severe.

With machines working on the rotary principle the stress on the framing is less severe; but very much depends in balancing all cutter blocks to the greatest nicety, as, should they be only slightly out of truth, owing to the immense centrifugal force at work, the adverse stress exercised on all parts of the machine is very great. All cutter-block spindles too should be of sufficient section to obviate all chance of springing even under the severest duty, and the spindle bearings should never be allowed much 'play,' or the vibration, which is often attributed to weak framings, is much increased.

As regards the design of the spindles and details used in the construction of wood-working machinery, if wrought iron is used, only that of the best quality should be employed, combining strength and toughness. This, when the bar is fractured, is shown by the fibres of the iron being close and uniform in the grain, free from whiteness or crystallisation, and of a bright bluegrey colour. The dimensions of spindles should be as uniform as possible, avoiding abrupt angles and sudden changes of diameter. Where spindles are subject to

much strain the alteration of diameter should be graduated by a curve, or what is known as rounded off, as any shock or vibration is thus more evenly distributed. For all the smaller diameters of spindles we prefer to use Bessemer or other mild steel, and the cost is very little in excess of the best wrought iron. It is of course rather more difficult to work, but this is

repaid by a finer surface being obtainable, as wrought iron is often found seamy, or, by the fibres of the iron not running parallel to the length of the bar, the abrasion of the surface, and consequent friction on the bearings, is much increased. In forging spindles, or any parts on which there is great strain, as few 'heats' as possible should be taken by the smith, as by constant reheatings the strength of the bar is considerably reduced. Great care also should be taken that collars or journals, or any forgings necessitating a number of heats, are not burnt, as we have more than once, especially in steel spindles, seen them from this cause break short off at the angle on receiving only a very moderate shock or blow.

CHAPTER XXXIII.

BEARINGS FOR WOOD-WORKING MACHINERY.

6

IN consequence of the high rate of speed at which it is necessary to operate wood-working machinery, the proper proportion, construction, and lubrication of the bearings whereby the friction necessarily engendered can be reduced to its lowest limits-is a matter of vital importance. Much time has been spent by scientific men in investigating the theory of friction, and many learned treatises written thereon. One of the most valuable of the earlier ones with which we are acquainted was written by George Rennie, F.R.S., and published in the Philosophical Transactions' for 1829. In this paper he gives briefly some of the deductions made by the earliest writers on this subject, including Amontons, who wrote in the year 1669, giving as his opinion that friction was not augmented by an increase of surface, but only by an increase of pressure, and that the amount was the same both with woods and metals when unguents were interposed. He likewise concluded that friction increased or diminished with the velocity, and varied in the ratio of the weight and pressure of the rubbing parts and the time and velocity of their motions. Most of the scientific men following Amontons agreed with his hypotheses, including De la Hire. Ex

periments as to the theory of friction were also made by Vince and Coulomb in 1784 and 1799. Following these, Euler concluded that it depended on the greater or less approximation of the asperities of the surface brought into contact by pressure, the resistance to which he agreed was one-third of the pressure; of the effect of velocities he was uncertain. Muschenbroek and others maintained that friction increased with the surface. Bossut divided it into two kinds, the first being generated by the gliding, and the second by the rolling of the surface of one body over another; he also concluded that it was effected by time, but that it followed neither the ratio of the pressure nor of the mass. A great number of other scientific men, inluding Lambert, Parent, Brisson, Camus, Schober, Meister, Leibnitz, Varignon, Bernouilli, Ferguson, Gregory, Leslie, Bulfinger, Young, &c., early in this century wrote on the theories of friction.

In the year 1784 De Vince made a number of experiments to determine the law of retardation, together with the quantity and the effect of surface on friction. He was of opinion that the friction of hard bodies in motion was an uniformly retarding force, but not so with soft bodies, such as cloth, which produced an increase of retardation with an increased velocity, and also that the quantity of friction amounted to about one-fourth the pressure, and that it increased in a less ratio than the weight of the body; that when the surfaces varied from 1.611 to 10-06 1 the smallest surface gave the least friction, and that friction was greatly influenced by cohesion.

A very elaborate series of experiments on the laws of friction were undertaken by Coulomb at the instance

of the French Academy of Sciences in the year 1799. He commences by examining the friction of plane surfaces gliding over each other, and divides it into two kinds, the first resulting from time and the second from velocity. The first he considers to depend on five different causes-(1) the nature of the bodies in contact; (2) the extent of surface; (3) the pressure on the surface; (4) the time the surfaces have been in contact; and (5) the state of the atmosphere. He was also of the opinion that the friction of wood on wood, and metal on metal without unguents, was in proportion to the pressure, which attained its maximum in a few minutes after repose, and that with heterogeneous surfaces, such as those of woods and metals gliding over each other, the intensity did not attain its limit sometimes for days. He also concluded that velocities had very little influence in augmenting friction except under peculiar circumstances.

In 1801 Mr. Southern, of Soho, made experiments on the surfaces of the spindles of grindstones running at a high speed, when he decided that, with rubbing surfaces moving at the rate of 4 feet per second over a length of surface of 1,000 feet, the resistance arising from the friction of 3,700 lbs. of load amounted to onefortieth of the weight.

Mr. George Rennie's experiments on the laws of friction were on a very extended and complete scale, but our space prevents our giving as lengthened a description as we should wish. His deductions, however, were as follows:-(1) that the friction of metals varies with their hardness; (2) that the hard metals have less friction than the soft ones; (3) that without unguents, and within the limits of 32 lbs. 8 ounces per square