222 KINEMATICS OF MACHINERY.
elastic bow itself. In this clever use of the elasticity of the bow, the principle of which may have been long anticipated by the primitive slingers, we certainly have an indication of a somewhat advanced development. The blow-tube, which is in use among the South American tribes in the form of the twelve feet long Sab- arkan, is younger than the bow, although certainly very old. With it both leaden bullets and feathered arrows are shot with great pre- cision ; as regards the kind of motion received by the projectile, the blow-tube is a precursor of our modern weapons.
In both these cases of the use of rectilinear motion a projectile is discharged, the motions of which come under the action of kosmic forces so soon as it leaves the machine, so that the purely machinal part of the motion is by far the smaller. Indeed the rectilinear motion, which appears so primitive to our geometrical ideas, occurs but seldom in the first growth of civilisation. The nearer a people were to their aboriginal condition so much the less do they appear to have been acquainted with motion in straight lines, so that again we have to learn that we must separate our judgment of what is really near and what really distant, from our own experiences of to-day.
In some of the instruments of war of the Greeks and Eomans (which, it may be noted, came from the East), the machinal side has already been distinctly developed, the storing of energy for throwing the projectile, especially, has been very effectually carried out. The cross-bow, which superseded the older form of the
weapon, formed in general the foun- dation of the ballistas and cata- pults; instead of the elastic bow, rigid arms were employed, along with wrenching springs (Fig. 172) of the kind described in 42, made from skin or hair.* The remaining FIG. 172. parts, guides, winches and gear
were arranged in a way that showed considerable skill both in design and construction.
It remains yet to be decided when the pair of elements screw and nut first made their appearance ; they were certainly known
- See W. Riistows and H. Kochly, Oeschichte des griech. Kriegywesens.
