It is estimated that there are over 4,000 different kinds and sizes of nails in market. Amongst the most important of these are: 1,common cut-nails; 2,finishing-nails,which are more slender and have not as large heads as common nails;

3, wrought-nails, used when it is necessary to clinch the nail;

4, clout-nails, which have broad heads and are used for nailing cloth, leather, sheet-iron, etc., to wood; 5, countersunk nails, in which the top of the head is flat; 6, billed nails, in which the head projects to one side.

The terms threepenny, sixpenny, etc., as applied to nails, arose from the fact that before cut-nails were invented all the nails in use were made by hand and sold by count. One hundred sixpenny nails were sold for sixpence (12 1/2 cents). Afterward, when competition had reduced the price, one hundred sixpenny nails were sold for a much smaller price. As soon as the cut-nails were brought out, the price fell so materially that the nails were sold by weight, but the old designations were still retained.

It is sometimes stated that the word "penny" is merely a corruption of "pound," and that "sixpenny" nails were "six-pound" nails, or six pounds to the 1000. This is mere imagination.

Bevan determined that a wrought-iron nail, 73 to the pound and 2 1/2 inches long, driven into dry elm to the depth of one inch across the grain, required a pull of 327 lbs. to extract it; and the same nail driven endwise or longitudinally into the same wood was extracted by a force of 257 lbs. The same nail driven two inches endwise into hard pine was drawn by a force of 257 lbs.; and to draw out one inch took 87 lbs. only.

The relative adhesion, therefore, in the same wood, when driven transversely or longitudinally, is 100 to 78, or about 4 to 3 in dry elm; and 100 to 46, or about 2 to 1, in pine; and in like circumstances, the relative adhesion to elm and pine is as 2 or 3 to 1.

The progressive depths of the same nail driven into hard pine by simple pressure were as follows: -

1/4 inch, by a pressure of 24 pounds. 1/2 " " " 76 "

1 " " " 235 " 1 1/2 " " " 400 "

2 " " " 610 "

To extract the same nail from a depth of one inch out of Dry oak required a pull of . . . 507 pounds. Dry beech " " " . . . 667 "

Green sycamore " " . . . 312 "

From these experiments we may infer that such a nail driven two inches into dry oak would require a force of more than half a ton to extract it by a steady pull. A common screw of one fifth of an inch in diameter was found to have an adhesive force of about three times that of a nail 2 1/2 inches long and weighing 73 to the pound when both entered the same distance into the wood.

Haupt, in his "Military Bridges," gives a table of the holding power of wrought-iron tenpenny nails, 77 to the pound, and about 3 inches long. The nails were driven through a one-inch board into a block, and the board was then dragged in a direction perpendicular to the length of the nails. Taking a pine plank nailed to a pine block with eight nails to the square foot, the average breaking weight per nail was found to be 380 pounds. Similar experiments with oak showed the breaking weight to be 415 pounds. With 12 nails to the square foot the holding power was 542 1/2 pounds; and with six nails in pine 463i pounds. The highest result obtained was for 12 nails to the square foot in pine, the breaking weight being in this ease 612 pounds per nail. The average strength decreases with the increase of surface.

Tredgold gives the force in pounds required to extract threepenny brads from dry Christiana deal at right angles to the grain of the wood as 58 pounds. The force required to draw a wrought-iron sixpenny nail was 187 pounds, the length forced into the wood being one inch. The relative adhesion when driven transversely and longitudinally is in pine about 2 to 1. To extract a common sixpenny nail from a depth of one inch in dry beech, across grain, required 167 pounds; in dry Christiana deal, across grain, 187 pounds, and with grain 87 pounds. In elm the force required was 327 pounds across grain, and 257 with grain. In oak the figure given was 507 pounds across grain.

From further experiments it would appear that the holding power of spike-nails in fir is from 460 to 730 pounds per inch in length, while the adhesive power of screws two inches long, 0.22 inch in diameter at the exterior of the threads, 12 to the inch, driven into half-inch board, was 790 pounds in hard wood and about half that amount in soft wood.

The force necessary to break or tear out a half-inch iron pin, applied in the manner of a pin to a tenon in the mortice, the thickness of the board being 0.87 inch, and the distance of the center of the hole from the end of the board l05 inch, was 976 pounds.

As the strength of the tenon from the pin-hole may be considered as in proportion to the distance from the end, and also to the thickness, we may for this species of wood - dry oak - obtain the breaking force in pounds nearly by multiplying together one thousand times the distance of the hole from the end by the thickness of the tenon in inches.