206 



♦ KNOWLEDGE ♦ 



[July 2, 1888. 



message, then say ^Bxy, writing down y ; c, writing down e ; 

 abc, writing down c ; r, writing down r ; ef, writing down/; 

 and so on, thus making the lomplete message : — 



1 3. mdmiijijecrjrrfudi^efvoccu. 

 The reading is similarly managed and quite easy. 



This cijiher is easy to write, easy to read, and cannot be 

 deciphered. It does not fulfil the fourth condition, which 

 Bacon notes as involved in certain cases. A message written 

 on this system openly proclaims itself a cipher. But so 

 does a message written on Bacon's system ; for though the 

 matter written be perfectly innocent and natural, the use of 

 two types at once suggests that there is more in the writing 

 than appears on the face of it. 



SIMPLE MECHANICAL TRICKS. 



(C on timied from page 179.) 



N the following trick (or experiment) the law 

 involved is that the centre of gravity of a 

 body tends always to seek the lowest possible 

 position. 



Provide two equal straight bai's of any con- 

 venient dimensions, a roller, shaped like an 

 oi'dinary rolling ruler, and another roller of 

 the form shown at R R' in fig. 5 — i.e., a double cone rather 

 elongated. Set the bars in the positions A B and A' B' 

 shown in fig. 5, the ends at A resting on the surface of a 

 table, while the ends B and B', thrown pretty far aj)art, are 



raised to such a height above the end A as to give a slight 

 slant (two or three inches, say, in a foot) to the bars. Then 

 set the straight roller on the slanted bars, and show that it 

 will at once roll down to A and thence on to and upon the 

 table — a movement which will by no means surpri.se the 

 observers. Then take the double-cone roller, and making 

 some remarks about modifying the action of gravity, s-et 

 this roller on the inclined bars as in the position RR'. To 

 the surprise of the observers, or at least of a considerable 

 proportion of them, the roller instead of travelling down to 

 A will seem to climb up the inclined rails, passing to B B' 

 and thence to the table. 



You may then take up the rods and the rollers, while 

 the spectators express their opinicjns upon what they have 

 seen. After a while remark that the roller evidently 

 climbed up the slope, out of contradiction, perhaps because 

 the slope was too steep ! Diminish the slope by taking a 

 book from each side and set the bars again sloping from the 

 lowered book-heaps, but taking care to set the bars this 

 time parallel to each other, the ends A and A' being set as 

 far apart as the ends B and B'. Roll the straight ruler 

 down as before, and then set the other across the bars. 

 Those among the audience who had not been able to under- 

 stand the rolling of R R' from A A' towards B B' before, 

 will see no reason why it should not roll that way as in the 

 former experiment. But instead of this it will now roll 

 towards A A' just as freely as the straight roller had done. 



The reason of these diflerent movements should be 

 obvious. When the rods are set in the position shown in 

 the figure, the roller R R' is really descending in moving 

 toward B B' ; for as it passes to the parts of the rods farthest 



apart it is supported on parts of its own surface, drawing 

 nearer to the points of the cones, and is more lowered by 

 this change than it would be raised, were it an ordinary 

 straight roller, by the slight slant of the rods B A and 

 B'A'. 



A pretty modification of this experiment may be obtained 

 by preparing a descent beyond B B' precisely matching the 

 ascent from A A' to B B'^the junction of the rods A B 

 and A' B' at B and B' with the descending rods from B and 

 B' to an apex like A on the further side, being close and 

 neat. For whereas a straight roller set on either slope 

 would pass downwards or away from B B', the double cone 

 roller will run up (apparently) to B B', and then, after 

 travelling a certain distance (the mere effect of inertia) on 

 what looks like the downward slope, will come to rest, and, 

 changing the direction of its motion, will pass back again 

 over the seeming ridge line at B B'. The double cone roller 

 will thus pass backward and forward over the top of the 

 system of slanted rods, in apparent defiance of gravity. But 

 in reality its centre of gravity is oscillating on either side of 

 its lowest position. 



An experiment depending on a kindred principle is 

 illustrated in figure 6. Each represents in section : fir.st, 

 two stout plane boards, A B and A' B', hinged (or otherwise 

 connected) at the ends A A' ; secondly, two friction-rollers 



Fig. C. 



R and R', easing the sliding of the flat board D D' in one 

 ease, and of the wedge-shaped block C C" in the other. In 

 the case illusf rated in fig. G, pressing the boards A B and 

 A' B' toward each other forces out the board D D' in 

 the direction shown by the arrow ; but in the case illustrated 

 in fig. 7, instead of the wedge C C being forced outwards 

 by such pressure, it is forced inwards, as the arrow indicates, 

 if the angle of the wedge is greater than the angle between 

 the pressure-boards. It is easy to see the reason of this. 

 In both cases the boards A B and A' B' yield to the pressure, 

 li and B', approaching ; but whereas in one case the motion 

 of D D' outwards makes room for this approach, in the other 

 the motion of C C" outwards would bring a thicker part of 

 the wedge between the rollers, and A B, A' B' would be 

 forced more apart by this than they would be drawn together 

 by the mere motion of the rollers outwards. 



Mechanical tricks depending on inertia are often very 

 surprising to the uninitiated. They are also very instructive. 

 It occurs to me to notice that I was once victimised most 

 unpleasantly by a mechanical trick of this sort played upon 

 me by chance, not by any practical joker. I was running 

 along a railway platfoim carrying >a heavy valise in one 

 hand, and a rather cumbersome but not heavy weight in 

 the other. My foot caught against some projection in the 



