idea, that we will not further discuss it. For all the heavier classes of machines, and some of those with a reciprocating motion, which in their working have a constant jar or vibration, such as steam mortising machines, we prefer the cored or hollow-section framing. This form of framing is doubtless higher in first cost, and somewhat more difficult to manipulate, but this is more than counterbalanced by its increased strength and resistance to stress, economy of material in ratio to strength, and increased compactness and neatness in design.

Much improvement in machine design has doubtless recently been made; but even now we frequently see large quantities of waste material in the framing or supporting brackets, which, if not absolutely injurious, is of no advantage to the working of the machine, whilst other parts, where strength is required and where a fillet' or flange in the casting would be of value, is left unprotected; the result is unequal strain, excessive vibration, and inferior work. The same remarks apply with increased force where the metal is scamped,' as in the so-called cheap machinery, where general efficiency is sacrificed to low first cost.

Although no absolute rules can be laid down, these defects, with a little practical experience, can easily be obviated, bearing in mind that the various proportions should be judiciously distributed, according to the strains to be put on them; that they should be convenient for the founder, and easily manipulated and renewable, in the first instance in the workshop, and afterwards in the saw mill. As regards the allowance to be made in constructing framing, in consequence of the constant additional vibrating strain it would vary

considerably in different classes of machines. In steam mortising machines with a reciprocating motion it would be as high as 25 per cent., whilst in band-sawing machines, where the strain is chiefly torsional and the speed or duty not especially severe, a considerably less percentage would be sufficient. In any case the working stress of framings should always be considerably less than the elastic stress, as the tangential and other strains produced by imperfectly balanced working parts, unequal or excessive duty, lack of speed, undue friction, uneven foundations, or inferior workmanship must not be lost sight of.

Great care should be exercised in the making and finish of the wooden patterns used; inferior woods should never be employed. The best yellow pine is suitable for the larger patterns, and mahogany for the small. Whatever little extra trouble or expense is incurred in making perfect patterns is amply repaid by the casting being much improved in symmetry and requiring less finishing. In machine framings, owing to the difficulty of retaining sharp outlines in cast iron, the profiles should always be bold and simple in design, the edges well rounded, and all hollows, where stability and extra steadiness in working is required, should be supported by suitable fillets or ribs.

Complex forms in machine design are in every way to be avoided. This may appear a somewhat trite, but it is certainly not an unnecessary, remark in these days of fierce competition and straining after originality, and young engineers may bear in mind with advantage the saying of James Watt that the greatest of all prerogatives is simplicity, but more especially when applied to machinery.'

If iron patterns are used, which is advantageous when a large number of castings are required, allowance must be made in the wooden pattern in the first instance for the contraction in casting in the subsequent cooling. It is somewhat difficult to determine accurately the amount of this contraction, which varies according to the size and thickness of the casting, the nature of the iron, and the rapidity of cooling; but for framing castings of medium size-say, up to two tons in weight-composed of iron of fair quality and cooled slowly, a calculation for contraction of about one-eighth of an inch per foot in either direction would, we think, be safe. The thickness of the metal in machine framings should always be as uniform or gradual as possible, so that the contraction of the molten iron in cooling may be tolerably equal. The sudden changes in a casting from thin to thick metal should be avoided, as they produce a degree of initial stress which lessens considerably its strength, strains and twists it out of shape, and at times even fractures it. Each part of a machine frame or details should, as far as possible, be calculated to bear easily the highest amount of straining action of whatever kind that is usually put on it whilst performing the severest duty of which the machine is capable. The exact calculation of these various and somewhat complex strains is, however, difficult, and must, after a certain point, be left almost entirely to practical experience. If motion or load is suddenly applied, the stress on both framing and wearing parts is much increased. This of course is especially noticeable in highspeeded machines; care therefore should be taken that the application of the belt or other power for producing motion should be very gradual. This is a point, we are

afraid, that is often overlooked, but one that adds considerably to the longevity of a machine. We have found the application of a quick-threaded screw for starting or stopping belts a simple and effectual method of gradually applying this power.

For machines where the speed is moderate and the duty light, framings with ribbed sections have some advantages, notably that of cheapness and easy manipulation; but on the whole tubular or cored framing, with its various modifications, is much to be preferred.

Care must be taken that the mixture of iron employed is of good quality, and produces sound castings free from air bubbles or honeycombs. When broken, the iron should appear close in the grain and of a light bluish-grey colour. Unsound castings, or those showing a mottled appearance, should never be employed where they may be subject to any considerable strain. The surface of the castings should be made as clean as possible, any little extra expenditure in this direction amply repaying itself in their appearance and strength. But how often do we see a pound a ton saved in the first cost more than spent in making the castings presentable by chipping them over their surface, reducing at the same time considerably their resistance. to stress by removing the outer skin of the iron. By casting the iron under pressure much sounder castings are produced, but the difficulties and cost of the process preclude its use, except for plain and well-defined articles and when expense is not of the greatest importance. Air bubbles or honeycombs may be detected, after a little experience, by ringing a hammer over the surface of the casting.

Especial care must be taken in designing the fram

ing of machines with a reciprocating motion, such as timber frames, mortising machines, &c., with a view of overcoming the excessive vibration. In America the main uprights of timber frames of large size are often constructed of wood, and attached to or combined with the timber supports of the building itself. By these means the jar and vibration is in a great degree absorbed, and saw frames constructed on this plan are found to work with a considerable amount of success. They are, however, open to objections for English use, and are on the whole somewhat cumbersome and inconvenient. Occasionally the heavier saw frames are arranged with an air cylinder, or to drive directly from a steam cylinder attached to the top of the frame. These plans have some advantages, and the vibration in working is considerably lessened, the air or steam acting as a cushion at each stroke of the swing frame. Latterly saw frames arranged with their crank shafts

to work on a level with the floor lines have been introduced. This is accomplished by making the vertical standards of the frame of unusual solidity, and attaching them and the working parts to a massive bedplate, or, in the case of light bed-frames, casting standards and bed plate in one piece. For light frames this arrangement is sometimes advantageous, but for heavy frames it is not to be recommended, and should never be pursued unless, from water or other causes, deep foundations cannot be obtained. Under all ordinary circumstances we are much in favour of a frame arranged with a pit crank-shaft, and whatever extra cost may be incurred in the way of foundations is repaid by decrease of vibration, which is absorbed by the masonry foundations instead of the mill floor. The work turned out is consequently cleaner.