Value of Zinc for Roofing. BICKNELL'S PATENT AMERICAN WINDOW WEIGHT PULLEY. Another Balloon Project. MODERN WRENCHES AND THEIR ANTECEDENTS. less cost.' chinery send from their first. introduction some kind of wrench was required to turn either the nut or the screw. Probably one of the earliest forms of the screw-head was that now in common use in jack-screws perforated ; the nut being fixedand the wrench being a simple lever thrust into the perforation. It is quite probable als that the first ap pliance for turning nuts upon a fixed screw was also a simple made for the purpose in the soon become apparent. The primitive form of the wrench for square nuts must be a matter of conjecture but it was probably either that known as the 'pot hook' wrench a bar of iron bent _twice at right angles or that form which has its jaws parallel to the shaft or handle and in which the nut is grasped so that the axis of the handle prolonged would pass through the axis of the screw instead of to one side as in the frook wrench., scientific american, 1869-02-20
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