420 



♦ KNOWLEDGE ♦ 



[Nov. 21, 1884. 



On October 3 the trials were made with j)owder charges 

 of 8oz., these charges being 12 in. in length, and being 

 covered with a 19A in. tamping of clay or .small coal. A 

 gun fired with clay tamping gave a length of flame of 10 ft, 

 with small coal tamping of 26 ft., as could be observed 

 through the windows. Then the floor was covered for 

 131 ft with a layer of coal-dust IJin. thick, derived from 

 poor coal from the Union mines of Horsbach, near Aachen. 

 When the guns were fired under these conditions with clay 

 tamping, the flames became 18 ft, and with small coal 

 tamping 31 ft in length. After this bituminous coal-dust 

 from Pluto mine in Westphalia was placed in a like manner 

 upon the floor and fired at, when a heavy explosion occurred, 

 the flame rushing forth 23 ft. from the mouth of the adit 

 level and thus reaching a total length of 190 ft. from the 

 head, and that without presence of any firedamp. A repe- 

 tition of this trial gave a like result. 



The experiments with fire-damp followed next. The 

 carburetted hydrogen gas was taken from a blower in the 

 mine, 394 ft below the surface above the Grolmann seam 

 from a coarse conglomerate, and conveyed in a pipe 

 3,608 ft. long to a gasholder on the surface, whence it 

 could be forced at will into the experimenting chambers of 

 the adit level A mixture of air with 5 per cent gas fired 

 at in the 20 cubic metre chamber with clay tamping, 

 showed a length of flame of 36 fc. The same mixture, with 

 a layer of coal-dust of only 65 ^ ft length, gave with clay 

 tamping a very heavy explosion with a flame of 171 ft 

 long, and much heavy after-damp. The violence of these 

 explosions may be gathered from the fact, that when coal- 

 dust from the Pluto mine, without any trace of fire-damp 

 in the adit, was fired at an iron tub or coal-wagon, weigh- 

 ing nearly 6 cwt., and standing outside before the adit 

 mouth upon a pair of rails rising 4degs., was pushed on 

 for 24 ft., and when fired with fire-damp as above it was 

 lifted from the rails and thrown a distance of 39 ft. In a 

 siding level a solid brattice work, 2 in. in thickness, was 

 entirely broken by the shock of the Pluto dust explosion 

 alone, and when rebuilt and fired at with dust and fire- 

 damp it was not only broken, but thrown a distance of 

 98 ft. 



It is to be hoped that these interesting trials, conducted 

 under conditions which enable experiments to be made 

 without danger, may be continued with all possible varia- 

 tions, and that they may yield results, from which all con- 

 cerned in coal-mining may derive benefit and complete 

 immunity from such explosions as are under investigation. 

 — Engineering. 



THE EARTH'S SHAPE AND MOTIONS. 



By Eichard A. Proctor. 



CHAPTER v.— THE EARTH'S ROTATION. 



(CoTitijvued/rom page 360.) 



THE general principles on which the properties of the 

 gyroscope depend are sufiiciently simple, though the 

 theory of rotating bodies is one of the most difficult sub- 

 jectE in the whole range of mathematical inquiry. Newton 

 himself shrank from attacking it, where, in dealing with 

 the phenomena of precession and nutation, he found it 

 directly involved. He preferred to regard the protuberant 

 mass of the earth's equatorial regions as a collection of 

 bodies travelling around the earth, and to consider the 

 influence of external attraction on the orbital motions of 

 those bodies ; and then having found that such and such 

 changes would be produced, he showed how far those 

 changes would be modified when the bodies, being rigidly 



attached to the earth, had to force her, so far as they 

 could, to participate in their peculiarities of motion. And 

 even modern mathematics, despite the wonderful power 

 which it gives us over the problems we have to deal with 

 in discussing the motions of the planets, yet leaves the 

 problem of rotating bodies one of enormous intricacy and 

 difficulty. 



However, for our present purpose, all that is necessary 

 i.s that we should understand the general principles on 

 which the theory of the gyroscope depends. 



First of all we must rememVjer that the figure of the 

 rotating disc has nothing to do with the observed pheno- 

 mena. A rotating sphere would exhibit them quite as 

 well, although there are reasons of convenience which 

 render the disc preferable. 



Secondly, we must dismiss the notion that gravitation is 

 primarily involved in the observed phenomena. Gravity 

 is a force conveniently applicable to exhibit the pecu- 

 liarities of the gyroscope, but any other force will serve 

 equally well. 



The fundamental property on which all the phenomena 

 exhibited by the gyroscope depend is simply this — that 

 when a body is rotating upon an axis, that axis tends to 

 maintain itself unchanged in direction, though free to take 

 up a new position jjaraUel to itself. Upon the speed of 

 rotation, and the mass of the rotating body, depend the 

 force with which the axis tends to maintain its direction 

 unchanged ; but let the body be ever so small and its rota- 

 tion ever so slow, some force is always required to change 

 the direction of the axis of rotation.* 



Now, it is not difficult to show that this peculiarity is 

 merely an expression of the fact that when a body is 

 moving in a given direction, it cannot be made to move in 

 a different direction without an expenditure of force pro- 

 portioned to the mass of the body and the velocity of its 

 motion- 

 Let me explain clearly how I mean this to be taken. 



If a body moves in the direction A B (Fig. 1), and we 

 wish it to move in the direction A C, we may effect this 

 by giving it an impulse in direction 

 A D, such that it would move from 

 A to D under that impulse in the 

 same time as it would take in moving 

 Fig. 1. from A to B if untouched. If it was 



moving very fast at first, it would of 

 course traverse A B in a very short time, and we must give 

 a very sharp impulse, because we are to force on this body a 

 proportionately rapid motion in the direction A D. Had 

 the body been moving slowly towards B, a slighter impulse 

 would effect our purpose ; but even then, some impulse 

 would of course be required. 



Now, if we consider how the different points in a ro- 

 tating mass are severally moving, we shall see why it is 

 that we find it so difficult to shift the axis of a gyroscope, 

 when the disc is in rai)id rotation. 



Let A B D (Fig. 2) be the circle described by any par- 

 ticle of the rotating disc, about the axis E E' ; and suppose 

 we want to shift the axis to the position e e'. This is 

 equivalent to making the particle travel in the circle 

 o B c D. Now at a and c, the particle would be travelling 

 in the same direction as before the change, so that no diffi- 

 culty arises here. But at the common points B and D of 

 the two circles, a distinct change of direction has to be 



* Foucanlt'a pendalnm experiments are in reality merely a case 

 of this great property, since a pendulum in swing is rotating about 

 a definite axis ; and if the whole change of position in the sup- 

 porting frame could be effected at the very instant when the 

 pendulum is at the limit of a swing, iindoubtedly the pendulum 

 would partake in the change of place. 



