Page:The Kinematics of Machinery.djvu/250

 228 KINEMATICS OF MACHINERY.

influences of the different epochs in which the machines have existed. We may note, however, that it has here been rendered possible by kinematic means for one man to work the machine instead of two or three.

We see something similar to this in the hand-spindle. Although force-closed in almost every direction we must yet consider it as a machinal arrangement, consisting of the elements spindle and thread (tension-organ) ; by it, however, the troublesome twisting of the thread, which before must have required, like rope-twisting, at least two persons, has been made possible for one worker, or at least the incomplete spinning of the Indians has been replaced by a quicker and better method.

In the Indian Picota, although this machine is of much later origin than the fire-drill, or even than the hand-spindle, we can trace similar processes. We have here forces of much greater amount to deal with,- there were Picotas with six or eight workers and correspondingly heavy buckets, but force-closure still governs the whole machine. There is force-closure in the main bearings, vertically by the weight of the beam itself, in other directions partly by cords but principally by the skilfully directed motion of the worker's feet; force-closure in the joints of the bucket-rod both top and bottom ; force-closure lastly in the body to be moved the water itself. The motions of the beam itself are regulated by a double force-closure, for of the two men one always presses downwards the end of it, while the other, maintaining his position by holding fast to his end of the bamboo frame, simply guides with his feet the end which is rising. In the Egyptian Shadoof the joint of the beam has become a wooden axis, pair-closure, that is to say, has come into use ; and by the use of a counter- weight it has been rende-red possible for a single man to work the machine with a not too heavy bucket by a suitable hanging rope.

In the long-bow, in which rectilinear motion is obtained by means which we have considered to be machinal, all the move- ments of the archer himself are foiH3e-closed ; we see also the bent finger guiding the arrow essentially by force-closure, while bow and string are alike force-closed elements. In the cross-bow and the ballista, a great portion of this force-closure is superseded, for the arrow is guided by a straight groove; the winding apparatus also is a kinematic chain which takes the place of the