ORIGIN OF THE SCREW-PAIR. 223
to the Greeks and Romans, as e.g. in carriage building, even if they were not very frequently used by them. In our antiquities the screw understanding this always as including the nut is exceed- ingly rare. The way in which what we call right-handed screws have been chosen so universally in preference to left-handed ones is ex- tremely remarkable, and is worth closer examination. I cannot attempt an explanation of it here, for too little material exists. It seems to me improbable that it can always have been as now, where people in general scarcely know of the existence of a left-handed screw. There are ancient representations of left-handed screws : besides several from the middle ages, there are those of fulling- presses in the Pompeian Fullonica which show both a right and a left-handed screw.
It is, meantime, very difficult to determine the way in which this notable pair of elements was arrived at. I cannot think the notion tenable that it was immediately suggested by some natural form, as for instance a spiral shell. There are indeed many cir- cumstances which seem to support this notion. First, that such shells, with few exceptions, have a right-handed twist. Then that in Greek the words for screw and for such a shell (/eo^X/a?, Ko%\iov, /eo^Xo?) are almost or entirely the same. Neither of these, how- ever, can decide the case. For once the screw had been found in whatever way it might very well on account of its form have afterwards received the name of the shell; the Greek word for spoon also, (tcoxXidpiov) has been derived from the name of the shell, obviously from its hollowness and not its spiral shape. The idea of a form immediately suggested by nature would presuppose such a leap forward in the course of machinal development as would entirely contradict all that we have observed, in every other case, of the almost sedimentary formation of ideas. Besides this, the shell gives a model of a conic screw and not a cylindric one, it requires translation into the latter form, the only one used. Before all, however, the shell gives no illustration whatever of the paired elements, with their definite relative motions and their striking capacity for exerting pressure.
The hollow screw, the nut, must have formed part of the pattern from which the idea of the screw motion was derived. I venture the conjecture I can hardly call it more that it was the fire-drill of Fig. 161 that led up to the screw. With long-continued
