seasoning wood by desiccation and other means have been tried, but nothing is found equal to the exposure of the wood to the atmosphere. When used for interior work, it should be kept for at least three years. Wood suffers decomposition from excessive heat, or if exposed to a humid atmosphere; but if buried, and the air entirely excluded, its decomposition is slow. Dry air does not seem to affect it.

The proper time and manner of felling trees, when they are at their prime for conversion, is a special business, and requires a special knowledge, that can only be gained by experience. In most timber-producing countries trees of all sizes are at present felled indiscriminately, but this pernicious system will doubtless in the course of years have its effect. We need hardly say that any tree showing incipient signs of decay should be immediately felled, as its commercial value will then rapidly decrease. The proper time for felling trees depends in a great measure on their genus; winter or summer is generally to be preferred, the timber being then less full of sap. Resinous trees are best felled in summer, other trees in winter, except oak or those from which the bark has to be stripped, in which case early spring is found the most suitable, as the bark will then peel most easily. The bole of the tree should always be severed as near the ground as possible. Although several attempts, with more or less success, have been made to fell trees by mechanical means, the difficulties of situation, transport, and economical manipulation have hitherto prevented much progress being made in thus superseding the art of the woodman. From experiments it has been found that the wood immediately surrounding the heart of the

tree is the weakest, and this weakness increases with the age of the tree. The woody fibres next the bark are also weaker than the rest, the strength of the timber gradually increasing from the heart outwards. Moisture in timber weakens it considerably, and its crushing resistance is found to be little more than onehalf of what it is when dry.

Experience, of course, can be the only sure guide as to the condition, strength, or value of timber; illconditioned timber can, however, generally be distinguished by a looseness and woolliness of the fibre and a clogging of the teeth of the saw; when cut it also presents a white, floury appearance. The timber used for joinery purposes and conversion into mouldings, &c., should be as sound and dry as possible. If it is necessary to season the timber artificially, what is known as the hot-air method is generally considered the best. This consists in confining the timber in a chamber, and subjecting it to a current of hot air forced through the chamber by means of a fan. It is necessary to vary the temperature of the hot air according to the kind and size of timber. The following have been found the most suitable drying heats:

Oak, the temperature should not exceed 105° Fahr.
Leaf woods

90° to 100°

. 120° 99

180° to 200°

. 280° to 300°

CHAPTER XXXVIII.

CIRCULAR AND STRAIGHT SAWS.

IN connection with the productive efficiency of woodworking machinery the proper selection, sharpening, and manipulation of the cutting tools employed is a point of the utmost possible importance, as, no matter how well designed or constructed a machine may be, unless the tools employed are exactly suitable to the work to be performed, are made of material of the highest quality, and are tempered and sharpened to the most correct cutting angle, the production of the machine is in every way unsatisfactory, the work turned out being less in amount and the quality inferior, whilst the force expended to produce it is greater.

As being the most general and important cutting instrument employed in the conversion of wood, we will first consider the saw. Saws may be divided into three classes-viz. (1) reciprocating or mill saws, (2) rotary or circular saws, (3) endless band or ribbon saws. Another form of saw is known as the cylinder saw, but this form is rarely used. On the gauge, shape of tooth, and 'set' of the saw best adapted to perform certain work a great diversity of opinion exists, English, Continental, and American practice on these points differing

considerably. No fixed rule can, however, be laid down, as all these points must depend on the nature, quality, and condition of the material operated on. An immense variety exists in the shape of the teeth employed; the best known are the peg, gullet, fleam, mill saw, hand saw, hog mane, dog, parrot bill, bird tail, hook, and American tooth. The first four mentioned are those chiefly employed in this country. In cutting or ripping with a circular saw soft woods, such as pine, with the grain of the wood, the teeth of the saw should be set farther apart, and the pitch or rake and set of the teeth should be considerably coarser than for hard wood. If a line is drawn through the points of the teeth the angle formed by the face of the tooth with this line should be, for cutting soft woods, about 65° to 70°, and for cutting hard woods about 80° to 85°. The angle formed by the face and top of the tooth should be about 45° to 50° for soft wood, and 65° to 70° for hard. It will thus be seen that the angle of the tooth found best for cutting soft wood is much more acute than for hard; these figures are, however, given approximately, and should be modified according to circumstances.

The action of the saw when operating with the fibre of the wood may be regarded as chiefly a splitting one, the revolving saw-teeth acting like a series of small wedges driven into and separating the longitudinal fibres of the wood.

Some saw teeth of American origin are very extraordinary in form, and the advantages gained by thus shaping them are not always obvious.

Mr. Peter Cook, an American, took out a patent some twelve years since for improvements in the shape of saw teeth. According to this invention the teeth

are made of a rectangular, or nearly a rectangular, form, the tops of the said teeth being bevelled to form a cutting edge throughout their entire length. The cutting edge of each tooth coincided with the line of motion of the saw, and such cutting edge may be of equal length to its base. The bevel of each tooth was on the opposite side to the next one, and the teeth may be set in the ordinary way, the set or deflection being on the opposite side to the bevel of the tooth. If desired the ordinary clearing tooth may be employed at suitable intervals along the saw. The principal advantages claimed to be derived from this construction of saws are smoothness left on the cut surfaces of the wood, the freedom with which they pass through the material operated upon, and the capability of cutting equally well in both directions.

We may here remark it is of the greatest importance that the steel used for the manufacture of saws and cutters should be of the highest possible quality; any advantage that may be obtained in lower first cost in purchasing a second quality is immediately thrown away in extra loss of time in sharpening, inferior work, &c.

In sawing resinous woods, such as pitch pine, the teeth of the saw should have a considerably coarser 'set' and space than for hard woods. For sappy woods saws with longer and sharper teeth should be used.

Before sharpening a circular saw it should be made perfectly round; this can best be done by placing the saw on the spindle and running down the points of the teeth by means of a hard piece of stone. It is important that the cutting angles and the tops and faces of the teeth should be bevelled exactly alike the gullets,