Page:The Kinematics of Machinery.djvu/619

 NOTES. 597

[I am glad to be able to point to at least one excellent elementary text- book, in which the results of experiment are " transferred to the text," Prof. Ball's Experimental Mechanics. The extent to which inaccurate statements on this subject are made by writers of ability, who are perfectly well acquainted with the physical facts in question, is really extraordinary. The following sentences , for instance, occur on the first page of a well-known treatise on Friction : (the italics are mine) "... The second class of forces, comprising the resistance of fixed obstacles, the resistance which fluid media oppose to the passage of bodies through them, and friction, are not capable of producing sensible motion, and show their effects only by the apparent destruction of the motion which has been caused by forces of the former class. These may be termed ' resisting ' forces. . . . Resisting forces are incapable of producing sensible motion, and if they do not actually destroy it they do convert it into a species of motion which is, to sight, insensible."]

23 (p. 203.) The series of phenomena adduced by Lubbock (Origin of Civilization, etc. Lond. 1870) in support of the theory of the unity of the human race (which is not to be confused with their growth from a single pair) is really extraordinary.

24 (P. 205.) Chamisso (iv. 244) gives the following description of this and other methods :

" In the Caroline Islands a piece of wood is fixed to the ground, and over it is held perpendicularly a second piece, about a foot and a half long, tolerably round, and of the thickness of one's thumb. This is caused to twirl by the palms of the hands, its lower and roughly pointed end being pressed against the fixed piece. The first slow regular motion is quickened and the pressure increased as the wood-dust, which is formed by the friction and collects round the borer, begins to carbonise. This dust is the tinder, and soon catches fire. The women of Eap possess wonderful facility in executing this process.

" In Radack and the Sandwich Islands they hold over the fixed piece of wood another which is about a span long and roughly pointed, and slope its upper end away from them at an angle of about 30. It is held with both hands the thumbs being placed below and the fingers above to improve the grip, and moved backwards and forwards in the plane of its slope through a distance of two or three inches. When the dust which collects in the groove formed by the friction begins to carbonise, the pressure and velocity are doubled.

" It is remarkable that in both methods the two pieces of wood used are of the same kind. They are best when of equally fine grain, not too hard and not too pliable. Both methods lequire practice, skill, and patience.

" The method of the Aleutians is the first of those above given, mechanically improved. They use the twirling-stick as they do the drill which they employ for other purposes. They hold and pull one end of the cord, which is twisted twice round it, with both hands ; its upper end is passed through a piece of wood prepared for the purpose, which they hold in the mouth. We have seen two pieces of fir-wood thus used give fire in a few seconds, a result which otherwise would have taken a much longer time.

"The same people also produce fire by striking together two stones rubbed with sulphur over dry moss strewn with the same material."

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