June 1, 1888.] 



♦ KNOWLEDGE ♦ 



179 



ing the character of the experiment — the selfsame conditions 

 which produce a more surprising result in the case illus- 

 trated in fig. 1. 



There is a familiar toy which represents a di-agoon 

 mounted on a galloping horse. This hoi-seman may be set 

 prancing on the edge of a table in the liveliest fashion with- 

 out any risk of an upset, and when so moving seems to pre- 

 sent a perplexing puzzle, even though the source of his 

 stability is displayed without anj concealment. The horse 

 and his rider are really kept in stable equilibrium by means 

 of a weight at the end of a curved wii'e which passes from 

 the stomach of the horse downward and backward, so that 

 when the horse is set upon his hinil legs at the edge of any 

 support the weight is under the hoise's hind feet. In this 

 position the weight is practically as the bob of a pendulum ; 

 the only diflerence is that the weight is connected by a 

 curved wire and the wooden figure of a horse with the part 

 resting upon the place of suspension, whereas the bob of 

 a pendulum is usually at the end of a straight wire. When 

 we set the horse galloping, we set the weight underneath 

 swinging, preci-sely as we might set a pendulum swinging ; 

 and there is no more chance of the horse upsetting than 

 there is of a well-swung pendulum jumping off from its 

 place of support. 



A number of experiments akin to the galloping dragoon 

 may be devised for exhibition without any special appai-atus. 

 Take for instance a heavy handled carving fork, a cork, a 

 strong needle, a flat-headed nail, and a piece of wood, 

 <lealing with them as shown in fig. 3. The nail is driven 

 into the piece of wood W, the fork F is driven into the 

 cork C, through which the needle n n' is thrust. The fork, 



Fig. 3. 



cork, and needle may then be safely set swinging on the 

 needle's point, which rests on the top of the nail, the piece 

 of wood being held horizonhilly as shown. The movement, 

 as the fork swings, is really that of a weight hanging from 

 a point of suspension, and is seen to be obviously stable as 

 soon as its true nature is considered. Yet it has a strange 

 effect to see the apparently unstable combination swaying 

 on a nail top, and on no better point of susjiension than the 

 end of a needle. 



In the last experiment the nail and piece of wood are 

 not necessary. A fork taken from the dinner-table, and a 

 cork probably found " handy by " (for even in these days of 

 prohibition pei-sons who can trust themselves and their 

 company not to get intoxicated, oceisionally have such an 

 infernal machine as a bottle of champagne, port, sherry, or 

 burgundy opened at their table), will, with a strong pin or 

 other representative of the needle, serve the purpose of the 

 experimenter sufficientlv. The combination can be set 

 swinging with the pin point resting on the flat handle of a 

 spoon, or even, if the experimenter is handy, on an end of 

 one of the prongs of a silver fork. 



Still simpler ways of showing the same illusti-ation of the 

 point maj' be suggested. Thus take a lead pencil, rather 

 bluntly pointed, and a parlour tire-poker, and tie the middle 

 of the pencil (say two inches from either end) to a part of 



the poker two inches from the working end. Then bringing 

 together the large end of the pencil and the end of the 

 poker, set the point of the pencil on the edge of the table in 

 the manner illu.strated in rig. 4. Then P, where the poker 

 and pencil meet, can be depressed so as to set poker and 

 pencil swinging on the point ;/ of the pencil, with perfect 

 freedom from all chance of an upset. In this form the trick 



Fig. 4. 



is particularly effective because the string s.v', which re- 

 places the rigid attachment of the other cases, seems in- 

 sufiicient to make the connection between the suspended 

 body and the point of suspension jj' effective ; and the pencil 

 and poker ai-e known to be free from any attachment at V p. 

 Yet when the poker is set swinging it is easy to see that 

 the pressure at P/>' and the tension along the string .« .«' 

 make the connection between the weight suspended and the 

 point of suspension suflicient as against all the forces which 

 are in operation. The weight of the pencil and poker 

 together acts vertically downward through the centre of 

 gravity G, and since this is below the point P the equi- 

 librium is stable. G oscillates under P, precisely as the 

 bob of a pendulum swung from P would oscillate. 



In these experiments we have simply a suspended body, 

 so shaped that when it is poised the centre of gravity is 

 hanging below the point of suspension. Thus every move- 

 ment given to the balanced body raises the centre of gravity 

 above its lowest portion, just as any movement given to the 

 bob of a pendulum raises it in greater or less degree. The 

 swinging motion illustrates the tendency of the centre of 

 gravity to seek the lowest position which it can attain. 

 This is the secret of all experiments in equilibrium. In 

 cases of stable equilibrium the centre of gi'avity tends to 

 rise under the action of external forces, and the force of 

 gravity brings it back, the body oscillating like a pendulum 

 around the position of rest ; in cases of unstable equilibrium 

 it will be found that the action of external forces tends to 

 depress the centre of gi'a\'ity, to which movement gravity 

 lends its aid, in such sort that the body moves farther and 

 farther away from the position in which it had been unstably 

 poised. The trick in the experiments above described con- 

 sists in so arranging matters that a position of stable 

 equilibrium is made to look like a position of instability. 

 (^To be continued.) 



The well-known firm of W. Watson Sc Sods, of 313 High 

 Holbom, occupied a very important position at the late Crystal 

 Palace Photographic Exhibition, and their exhibits attracted con- 

 siderable attention from the trade and amateurs alike. They were 

 awarded two medals, one of them for cameras and fittings (with 

 special commendation for the introduction of interchangeable parts) 

 and the other for tripods and studio stands. 



Messrs. Marios ct Co. were also very successful, and obtained a 

 medal for appliances for artilicial illumination, another for mounts. 

 a third for albums and cases, and a fourth medal for general 

 appliances and plant. 



