Mr. Bevan experimented on iron wood-screws 2 inches long, 22/100diameter at exterior of threads, threads 35/1000 deep, 12 to the inch. These were driven into boards 1/2 inch thick. The force required to extract them was - from hard woods about 790 lbs., from soft woods about half that amount.1

Fig. 188.

Making Screws is a subject which is beyond the scope of these Notes.

Wood screws ordinarily used by the carpenter and joiner are made by machinery, the thread being turned in a sort of lathe. Very large screws are also turned in lathes in ordinary workshops.

Small metal screws are cut in screw plates, larger ones with stocks and dies; and the threads to receive screws may be tapped by the aid of hard master taps, turned by means of a long double handle.

Bolts and Nuts are a good deal used by the carpenter for heavy work, and are also required in connection with iron roofs, etc.

They hardly come within the range of notes on materials, and it i& impossible, for want of space, to describe them at all.

l Tredgold.