This operation is performed by the vertical descent of a ponderous arm of iron, terminating in a cutter of the form of the notch to be made in the yet soft and smooth edge of the circular plate, which is made by the same power to revolve horizontally upon an axis placed at such distance from the impinging weight as the depth of the notch to be cut requires, and traversing at a rate so timed in unison with the descent of the cutters as to render the series of teeth perfectly continuous and equal; each blow of the cutter forming the interval between two teeth, and each full revolution of the plate completing a circular-saw. In the same way is effected the teething of the straight saws, the motion being a direct sliding action in a forward line, instead of a rotatory movement. In the English saw works, owing to the influence of trade-unions, operative-unions, and the like, the application of steam-power to this machinery is prohibited, and the employer is restricted to the use of hand labor—the cutter being jerked down by man power, and the edge of the plate to be cut being subjected to the striker by hand, the formation of the teeth not being regulated by any absolute scale, but being executed by the calculation or guess-work of the artisan, and, of course, varying in accuracy, depth and precision of cutting according to the skillfulness or unskillfulness of the individual operator. To the absence of these ingenious combinations, injurious alike to the true interest of operators and employers, the superiority in many respects of American to English machinery is in some degree due, and not less to the over stringency of the patent laws of Great Britain, which often prevent the application of really leading and most material improvements, of a radical nature, to principles secured for the benefit of the inventor. We may here observe that the use of circular saws is very greatly on the increase in this country, more especially in the western portion of it. In the east, for some inexplicable reason, this admirable instrument is far less generally used; and the writer of this article, several years ago, when on a visit to the timber districts of Maine, on expressing his surprise at the non-adoption of this most excellent and labor-saving tool, could learn no adequate cause for the prejudice existing against it, unless it were some crude and absurd ideas concerning its vibration and consequent irregularity of cutting—objections not founded on facts, nor confirmed by experience. From the saw shop the circular plates, now teethed and in the incipient stage of what Willis would call _sawdom_, are removed across Broome Street into the other building, and introduced to the saw hardening room, where they are converted into highly tempered steel. [Illustration: SAW HARDENING ROOM.]