a well-known instrument for cutting wood.

The saw is of unknown antiquity, being figured on the ancient monuments of Egypt and Babylon. It is probable that saws were first made of copper, hardened by the admixture of a little tin. But after the invention of steel that substance has come, almost exclusively, to be used in their manufacture.

The common saw consists of a steel plate, on the edge of which teeth are formed; these teeth, when drawn over a piece of timber, act as a succession of chisels, and so make a cut or saw-draft. When the teeth are left in the plane of the plate, and when the saw has entered a little way into the timber, the sides of the cut come to rub against the plate, thus causing a friction, which soon arrests the operation. There are two ways in which this evil is remedied: the one is by making the plate thin towards the back, as in the key-hole saw; the other is by bending some of the teeth to the one side, some to the other side of the saw-plate, so that they make a draft considerably wider than the thickness of the plate. This bending is called the set of the saw. It is obvious that the best method is to set the teeth alternately to the right and to the left; but in the East, where ancient practices are scrupulously adhered to, the teeth of the large saws are bent aside in groups of perhaps a dozen each. The quantity of set is regulated by the nature of the material to be cut; for soft and fibrous woods, more set is needed than for hard woods. Since the labour of cutting and the waste of material are increased by the set and consequent width of the saw-draft, it is an object to give to the saw as little set as possible, and the friction is reduced in some cases by bending the halves of the log asunder by means of wedges; in other cases by the use of soap, tallow, or oil, to lubricate the sides of the saw.

Although it seems to be a very simple affair, the sharpening and setting of a saw require considerable skill of hand and accuracy of eye, for if any one of the teeth project either edgewise or sidewise, beyond the general line, it takes hold of the wood and renders the sawing harsh and difficult. The form of the tooth, also, is a matter of some importance. When the teeth of the saw become blunted by use, they are sharpened by means of a file, which is three-sided, or, as the workmen call it, three square. This is the most economical form of saw-file, because all its edges and faces are equally available. The angles of a four-sided file are too great, and an isosceles three-sided one would, although the angle might suit better for some purposes, have only one or at most two edges available. Hence for common use we are compelled to adopt the ordinary equilateral three-sided file, and, in consequence, the angle of the saw-tooth is sixty degrees.

Let ABCDEFGHI represent part of the edge of a saw, of which the notches BCD, DEF, &c., have been made by a three-cornered file; then if the sides AB, BC of the tooth be equally inclined (as in fig. 1) to the general line of the edge, the instrument acts equally well in either direction; but in this case the teeth can hardly be said to cut, they only scrape away the material. Saws of this kind are used in cross-cutting, being worked by two men—one at each end; but as the teeth are large, they are punched out at the fly-press, and are made deeper than an equilateral trigon.

In order that the tooth have a proper cutting edge, the front BC must be (as in fig. 2) at least perpendicular to the line of the cut, and therefore the back AB must, at most, make an angle of thirty degrees with that line. For many kinds of timber it would be advantageous to have the front of the tooth leaning forward (as in fig. 3), but this form allows little room for the saw-dust, and is accompanied by a tendency to drag the tool in upon the wood, thus causing the strain to be too great for the hand-saw; nevertheless, a slight inclination forward is, in certain cases, advantageous.

When a saw is to be sharpened, it is convenient to draw the file gently along the tops of the teeth, and so reduce them all to an even line; we then take care, in filing out the notches, to do no more than remove the bright surfaces thus exposed: by this precaution we secure the even lining of the teeth edgewise. The setting is a more difficult affair; it is done sometimes by the hammer and sometimes by a saw-key. When the hammer is to be used, the saw is laid upon an iron plate, so that the teeth may project over its chamfered edge. Each alternate tooth is lightly struck by a hammer, the weight of which must be adapted to the size of the teeth; the saw is turned over and the intermediate teeth struck on the other side; the lining is examined by the eye, and any errors are corrected. The setting key is a piece of steel, having notches of various widths cut in it to suit the different thicknesses of saw-plates; by means of these notches the teeth can be bent to the required degree. It has been proposed to fix a stop upon the set-key, which stop, coming against the plate of the saw, may regulate the degree of bending; but the elasticity of the steel carries the tooth back somewhat from the extreme angle of flexure, so that the residual set depends partly upon the temper of the steel; thus, after all, the eye becomes the ultimate judge.

In filing the teeth we must pay attention to the set; thus, if the tooth B be bent to the left, the left edge of the face BC should be made acute, as should also the right edge of DE. Hence the alternate spaces BCD, FGH, &c., should be filed out from the right, and the intermediate spaces, DEF, &c., from the left, the point of the file being, in both cases, inclined a little forward.

The saw-plate should be left as hard as is consistent with its being bent; if it be too hard, the teeth are apt to break off in being set; if it be soft, the teeth are readily blunted.

To the one end of the saw-plate a handle is fixed, by means of which it can be moved backwards and forwards. In this country it is usual to make the saw cut when being thrust forward; in other words, the teeth look from the handle; but the modern artificers in Greece and other eastern countries make the cut in pulling, as also did the ancients; the teeth look towards the hand. There is something to be said in favour of and against both methods. The cut, in pushing, enables us to apply greater muscular energy, and the entrance of the saw being towards the eye, we can readily follow the line; but, on the other hand, any sudden jerk or unskilful holding of the tool is apt to bend or buckle it. The cut, in pulling, again, tends to straighten the saw, so that a very thin plate may be used, requiring less labour and causing less waste of material; it is peculiarly suitable for workmen, who hold by their toes; but then the saw-dust is heaped upon the line and has often to be swept or blown away.

The advantages of both of these methods are obtained by stretching the saw tightly in a frame; and even more than these advantages, because we can use saw-webs much narrower and much thinner than when the plate is to be held by one end. The narrowness of the web allows it to be turned considerably within the saw-cut, so that it may follow a curved line, whereas the hand-saw is almost restricted to a straight cut. Saw-webs for frames are made of great minuteness, the breadth even so little as the fiftieth of an inch, and the width of the draft not perhaps the third of that. These hair saws are extensively used for inlaying woods, ivory, and shell, as they can follow the most delicate outlines; they are also used by gold and silver smiths, on account of the smallness of the waste which they occasion.

Saws for cutting bone, ivory, or brass, require very little set; it is sufficient to have the web somewhat thinner toward the back. For these saws the steel is left pretty hard, as much so, indeed, as is consistent with their being readily filed. The preparation of saws for cutting iron or steel is a matter of considerable nicety; they must be very hard in order to resist the oxides and silicates which are to be found in the most carefully prepared iron, and yet they must be so soft as to be filed, although with difficulty; hence really good iron-saws, as they are called, are somewhat scarce.

The teeth of large saws, such as those used in the saw-pit or in the saw-mill, are cut out by means of a fly-press, in which a punch and socket exactly of the form of the notch are fixed. The edges are afterwards dressed by the file. Hence they are shaped more in accordance with the principles of cutting tools. The front of the tooth is hooked forward by even so much as forty or forty-five degrees, and the back is relieved only by an angle of eight or ten degrees. But if lines with these inclinations were continued to meet each other (as shown in fig. 4), there would be no room left for the dust, and the saw would be immediately clogged; wherefore a large hollow space is scooped out (as shown in fig. 5), giving to the teeth a very formidable appearance. These saws are sharpened by filing the parts marked A B; and when the teeth become too shallow, they are again deepened at the fly-press, or, if no fly-press be at hand, by means of round files applied to the hollows.

Circular Saws.—The straight saw must necessarily have a reciprocating motion; but when teeth are formed on the edge of a circular disc, the motion is continued in one direction, and great velocity can be obtained. Circular saws are now so very generally used that every one at all interested in mechanical affairs must be familiar with their general appearance and mode of operation.

Part A B C of the circular disc projects through a slit in the table D E. The piece of timber to be cut is laid on the table, and carefully brought up to the descending part of the saw at C; the saw being made to revolve with great velocity. When slices of a given thickness have to be cut off a log, a fence is placed on the table at E, parallel to and at the proper distance from the plane of the saw: the log is laid against this fence, and is pushed forward either by hand or by help of the machinery.

This is a dangerous instrument, not to be approached without caution, and, above all, not to be tampered with by foolish meddlers. If a lump of wood were presented at the rising part A, it would be caught by the hooked teeth, hurried over towards C and thrown off, to the great danger of those around, as well as of the saw itself.

Small circular saws have their teeth shaped by the triangular file: the fronts of them should point to the centre.

Veneer Saws.—The veneer saw is contrived for the purpose of cutting finely variegated woods into thin broad slices. The veneer varies from a 24th to a 30th of an inch in thickness, so that if the saw-draft be one 24th of an inch wide, half of the timber is cut into saw-dust. Now, the wood of which veneers are made is almost always very valuable, and hence it is of great importance to have the saw extremely thin.

The body of the veneer-saw consists of a lenticular piece of cast-iron, having the meniscus form—that is, hollow on the one side and round on the other. A section of it is given in fig. 7. The hollow part at B is made sufficiently deep to receive the bush which carries one end of the axle, and which rests on an overhanging support. Part of the edge AC is turned flat, and to this flat part the steel saw is screwed in segments—the screw heads being countersunk so as not to project beyond the outer face of the segments. These segments are ground away toward the edge, as shown in the section K L, and are thus made to form the rim of a very thin circular saw. The grinding and adjustment of these segments require great care.

The log to be cut into veneer is secured to a frame F, which can be moved by screws G, transversely upon a carriage E. This carriage moves upon two long iron slides or rails, laid exactly at right angles to the axis of the saw. The screws G serve to regulate the thickness of the successive slices. When the log is advanced upon the saw in the direction of the arrow, the thin veneer, which has been separated at the edge I, is bent away over the back of the saw, either by the workman's hand or by a small guide set up for the purpose. When one cut has been made, the carriage is returned, the log moved transversely by means of the screws G, and then advanced again to meet the saw.

Veneer saws are made from 5 to 20 feet in diameter, according to the breadths of the logs which they may have to cut, and from 12 to 18 slices are made in each inch of thickness.

Cylindric Saws.—The cylindric saw is formed by making teeth on the end of a thin tube of steel, and are useful for cutting circular holes in valuable material, as ivory or tortoise-shell, without destroying the interior part. As there is no way for the escape of the cuttings, they have to be withdrawn now and then from the cut.

Ribbon Saws.—The latest novelty in the formation of saws is the ribbon-saw, which is formed of a long, thin web of steel united at the two ends, or, it may be, of a continuous web rolled from a steel ring. This web, having teeth cut in one of its edges, is passed over two large pulleys, which are put in rapid motion; the descending free part of the web acts as a straight frame-saw, with this difference, that the motion is continuous, and that the action is very rapid.

Lapidary's Saws.—For cutting slate and other soft stones saws of steel are used; but for harder stones, webs of soft iron or copper, primed with sand and water, are employed. It is by means of such a tool that marble is sawn into slabs. When the plate of iron, stretched in a proper frame, is moved backwards and forwards upon the stone, the sharp fragments of the sand imbed themselves in it and act as saw-teeth, but they are so soon rubbed out that the action partakes greatly of the nature of grinding.

For very hard stones emery and corundum powders are used, and for extremely hard stones pounded diamond is employed; the circular form of the saw being generally adopted. The diamond-wheel is truly a saw, for the fragments of diamond stick with great tenacity into the iron, and act exactly as saw-teeth.

Lapidary's saws were well known to the ancients: some of the sarcophagi retain the marks of the tools by which they were scooped out. A tube of iron or of copper, primed with sand and water, was turned round upon the stone, and thus caused to make a cylindric cut; when this cut was sufficiently deep, the central cylinder of stone was broken over. Another and another hole of this kind was made until the required hollow was roughly formed; the intermediate triangular pieces were broken away, and the inside of the chest was then finished by the chisel and smoothing tools. This is exactly the process which is followed at the present day in doing the like kind of work.