The scissors and shears to be described in this and the succeeding section, act on a very different principle from the nippers recently spoken of. The nippers have edges of about 30 to 40 degrees, meeting in direct opposition, but yet leave ragged edges on the work; whereas the shears have edges commonly of 90 degrees, seldom less than 60 degrees, these edges pass each other and leave the work remarkably keen and exact.
Let the edges of scissors be ever so well sharpened, they act very imperfectly, if at all, unless the blades arc in close contact at the time of passing; and this imperfection is the more sensible the thinner and more flexible the material to be cut, as it will then fold down between the blades if they do not come in contact. Whereas when the blades exactly meet, the one serves to support the material whilst the other severs it; or rather this action is reciprocal, and each blade supports the material for the other, fulfilling the office of a counter-support, or of the bench, stool or cutting-board, used by the carpenter with the paring chisel.
On a cursory inspection of a pair of ordinary scissors, it may be supposed that their blades are made quite flat on their faces, or with truly plane surfaces like the diagram fig. 910 overleaf, representing the imaginary longitudinal section of the instrument, the two blades of which are united by a screw, consisting of three parts differing in diameter, namely the head, the neck, and the thread; the bottom of the countersink that receives the head of the screw is called the shelf or the twitter-bit. If however the insides of scissors were made flat, and as carefully as possible, they could scarcely be made to cut slender fibrous materials, or if at all, then for only a short period, and additional friction would accrue from the rubbing of their surfaces.
The form which is really adopted, more resembles the exaggerated diagram fig. 911; the blades are each sloped some 2 or 3 degrees from the plane in which they move, so that their edges alone come into contact; instead of the blades being straight in their length they are a little curved so as to overlap; and close behind the screw-pin by which they are united, there is a little triangular elevation, insignificant in size but most important in effect, which may be considered as a miniature hillock or ridge, sloping away to the general surface near the hole for the screw. This enlargement or bulge is technically called the "riding part," and as there is one on each blade, when the scissors are opened or that the blades are at right angles, the points or extremities only of the riding parts come into contact, and the joints may then have lateral shake without any prejudice. But as the blades are closed, first the bases or points of the riding parts, and lastly the summits or tops, rub against each other, and tilt the blades beyond the central line of the instrument; the effect of which is, to keep the successive portions of the two edges in contact throughout the length of the cut, as by the time the scissors are closed, the points of the blades are each sprung back to the central line of the scissors, which is dotted in the diagram.
Although scissors when in perfect condition for work, may be loose or shake on the joint when fully opened, (and thereby placed beyond their range of action,) they will be always found to be tight and free from shake, as soon as the blades can begin to cut the material near the joint, and so to continue tight until they meet at the points. That all scissors do exhibit this construction may be easily seen, as when they are closed and held edgeways, between the eye and the light, they will be found only to touch at the points and at the riding parts, or those just behind the joint screw, the remainder being more or less open and gently curved; and their elastic action will also be experienced by the touch, as whilst good scissors are being closed, there is a smoothness of contact which seems to give evidence of some measure of elasticity.
Fig. 912, represents the section of the one blade of a pair of scissors registered in July 1841, by Mr. G. Wilkinson, of Sheffield, and in which the elastic principle is differently introduced. These scissors are made without the riding part, but instead thereof, immediately behind the screw which unites the blade as usual, the one blade is perforated, for the purpose of admitting freely, a small pin or stud fixed to the end of a short and powerful spring, so that the stud s, from acting on the opposite blade throws the points of both towards each other, so as to give them a tendency to cross, but which being resisted by the edges of the blades touching one another, keeps them very agreeably in contact throughout their motion, and causes them to cut very well.
If further evidence is wanted of the clastic principle in scissors, it is distinctly shown in sheep shears, which besides their ostensible purpose of shearing off the fleece, are used by leather dressers and others. It is well known that sheap shears, fig. 919, page 915, are made as one piece of steel, which is tapered at each end to constitute the cutting edges, is then for a distance fluted and straight to form the semi-cylindrical parts for the grasp, and that in the center or opposite extremity, the steel is flattened and formed into a how by which the blades are united and kept distended; sheep shears consequently require no joint pin, and the hands have only to compress them as they spring open for themselves. If sheep shears are examined when fully opened, or when partially closed by tying round the blades a loop of string, it will be found that the blades have a tendency to spring into contact, as after having been pressed sideways and asunder, the cutting edges immediately return into exact contact the moment the distending pressure is removed.
The construction of scissors with the riding place as adverted to in fig. 911, is that which ordinarily obtains in most scissors, from the finest of those used by ladies, to the heavy ponderous shears for tailors, which sometimes weigh above six pounds, and are rested on the cutting board by one of their bows, that are large enough to admit the whole of the fingers.