224 KINEMATICS OF MACHINERY.
use, the cord may have worn or pressed spiral grooves in the spindle, and these formed in a manner screw-threads, the envelop- ing cord itself being the nut. The frequency with which this accidentally formed screw action was observed may have led gradually to its useful employment. The forms of the word screw in the Germanic languages greatly strengthens my suggestion. We cannot take into account the fact that in English and the Eomance languages, the characteristic portion of the screw is still called " thread" (filo, filet), for this name may have been subsequently given to it. It is at present difficult to see, although it may in time become clear, whether the screw was first used for causing forward motion, or for a fastening, or for exerting pressure ; it is difficult also to say in what manner the nut was originally formed. It is for the philologist, as the explorer of primitive times, to solve this question as to the original form of the screw and nut.
A variety of force-actions and causes slowly developed them- selves in the machine, besides the ever-increasing variety in its motions. In taking the fire-drill, in which the expenditure of force is comparatively trifling, as the first machine, we directly con- tradict the very popular notion that the lever occupied this position. Apart from the fact that it is by no means clear what precisely is to be understood by the term lever, this notion shows in my opinion a mistaken idea of the way in which human capacities have generally grown, and must have grown from the first. In taking the lever as the first machine, we think of men's attempts to deal with or overcome great resistances. It is not this, however, which first attracts the opening consciousness, but is much rather the accompanying phenomenon, motion. The child shows the most lively interest in the sails of a windmill, in the mill-wheel and such other portions of machinery as have distinctly regular motions : at first, however, he thinks nothing of the forces applied or brought into action by them. The separation of the idea of force from that of motion is a very difficult mental operation, and we find it occurring late and gradually. We find accordingly that the machines coming first from the unaccustomed hands of their makers are those in which forces play a comparatively subordinate part, for they do not exceed the exertions which the worker himself makes, imperfectly conscious of what he is doing.
This really lies at the root of the continual recurrence of the
