590 



NATURE 



[Oct. 2 1, 1880 



pounds on each square inch of surface. M. Tissandier, 

 in describing this simple experiment in the pages of La 

 Nature, has suggested that a few imprisoned flies or other 

 insects may without any cruelty or hurt do duty as divers 

 within the miniature diving-bell, and afford proof that life 

 can go on in the inclosed air even though below the 

 surface of water. 



In speaking in a former article of the subject of incj-tia 

 we mentioned the following familiar trick : a number of 

 the round wooden "men" used in playing the game of 

 draughts are piled up in a column one upon another. If 

 the lowest one of the pile is dextrously hit with the edge 

 of a paper-knife or other suitable article it may be 

 knocked away from under the others without overthrow- 

 ing the others. Fig. 27 shows how the experiment is 

 arranged, the narrow slip of wood which serves as the lid 

 of the box being here used as the weapon. Beginners in 

 science must not mistake the meaning of the term inertia 

 as applied to matter. Matter is not in itself lazy or inert. 

 But it possesses the property of mass, and to set mass in 

 motion requires the expenditure of ctiergy. If we skilfully 



a'N- 



spend the energy of the rapid blow upon the one draughts- 

 man, it is knocked away before there is time for any con- 

 siderable part of the energy to be imparted to the others 

 that are piled upon it. 



Another simple experiment, depending partly upon the 

 inertia of matter and partly upon elasticity, is often shown 

 as an after-dinner trick. Upon a linen tablecloth is 

 placed a threepenny-piece between two pennies or other 

 larger and thicker coins. Over this an empty wine-glass 

 is placed, and the puzzle is how to get out the smaller coin 

 without touching the glass. The very simple operation of 

 scratching with the finger-nail upon the cloth, as shown 

 in Fig. 28, suffices to accomplish the trick, for the little 

 coin is seen to advance gently towards the finger until it 

 has moved completely away from under the glass. The 

 fibres of the linen cloth are elastic ; when you scratch with 

 your finger-nail they are drawn gently forward until the 

 force of their elasticity becomes too great and they fly 

 back, to be once more drawn forward, again to slip back, 



and so on. While the fibres are drawn forward slowly, 

 they drag the coin with them to a minute distance. But 

 when the slip occurs and they fly backward, they do so 

 very rapidly, and slip back under the coin before there is 

 time for the energy of their movement to be imparted to 

 the coin to set it in motion. So the coin is gradually 

 carried forward over the surface of the cloth. 



We will next give a simple experiment which illustrates 

 the principle that a substance which is very weak in one 

 direction may be very strong in another, the "strength" 

 of the material (that is to say, the resistance it offers 

 before it will break) depending on the way in which aJ 

 force is applied to it. It is possible to lift a decanter fulll 

 of water by means of a single straw. To do this the-1 

 straw must be bent as Fig. 29 shows, so that the weight 

 comes longitudinally upon the straw. The straw is a 

 very weak thing if it has to resist a force applied laterally. 

 Lay a single straw horizontally, so that the tivo ends are 

 supported, and then hang weights on to the middle of it: 

 a very few ounces will break it across. But let the 

 weights be fixed to one end of the straw, and the straw 

 itself be hung downwards so that the pull is exerted along 

 it, and it will support one or two pounds at least. When 

 bent, as in the figure inside the bottle, most of the weight 

 is applied as a thrust against the end of the straw ; the 

 bottle tilts slightly until the centre of gravity of the whole 

 is below the point from which it hangs between finger 

 and thumb ; but in this position the sideway thrust 

 against the middle of the straw is very small, and the 

 material is strong enough to stand the strain to which it 

 is subjected lengthways. 



Lastly we 'offer an illustration (Fig. 30) of an experi- 

 ment known to every schoolboy. A can or jar containing ' 

 water may be whirled round the head without spilling a 

 single drop, provided the motion be sufficiently rapid. 

 When a moving body is subjected to the constraining 

 action of a force which acts, like the pull of a string held 

 in the hand, towards a fixed centre, the result is a motion 

 around the centre of force. Were it not for the con- 

 straining force the moving body would fly away at a 

 tangent ; and to the reaction exerted successively in all 

 directions away from the centre the name " centrifugal 

 force" was formerly given. The water in the can, being 

 heavy, is also subjected to this so-called centrifugal force 

 as it moves around the fixed centre, and hence it does 

 not fall out of the can while passing through the inverted 

 position in the air if its speed be sufficiently accelerated. 



THE GERMINATION OF VVELWITSCHIA 

 MIRABILIS 



HAVING been supplied, through the kindness of the 

 Director of Kew Gardens, with young seedlings of 

 Wclivitscliia miraliilis, I have been enabled to draw 

 some fresh conclusions as to the homology of the large 

 leaf structures, which have hitherto been described as 

 persistent cotyledons. It is true this description has 

 been confessedly provisional, since the process of germi- 

 nation has not hitherto been traced. 



The seeds germinate in a manner corresponding in the 

 main with that described by Strasburger for Ephedra 

 liinipylopoda (" Conif. und Gnet.," p. 320). The radicle 

 first breaks through the testa, the point of perfora- 

 tion depending apparently upon the position of the seed 

 during germination. The cotyledons also break through 

 the testa, but at a different point from the radicle. The 

 cotyledons are two in number; in one case I observed 

 three, one being smaller than the others. They free 

 themselves entirely from the seed, and expand to a length 

 of I inch to I A inch, with a breadth of \ inch, or rather 

 more ; it is possible, however, that they may by growth 

 attain a larger size. The cotyledons when expanded are 

 green, though while still in the seed they are yellow. 



