94 



KNOWLEDGE. 



[April 1, 1895. 



points at some two thousand miles down in the bowels 

 of the earth. This shows that the centres of the earth's 

 magnetic action are deep-seated. The term magnetic poles 

 for places on the earth's surface is perhaps somewhat mis- 

 leading. These positions are not poles in the sense in which 

 the term pole is used in connection with a bar magnet or a 

 magnetic needle. In these cases we mean positions near 

 the ends, where there is greatest concentration of magnetic 

 strength, centres of force in the magnet. The dip of 67° 

 in these latitudes shows, as has been said, that the centres 

 of magnetic force in the earth are deep-seated. If there 

 be "poles" comparable to those of the poles of a bar 

 magnet, they must be even further from the surface of the 

 earth than the point at which the direction of the 

 Greenwich dipping needle meets the direction of the needle 

 where the dip is 90°. 



The strength of the earth's magnetic force has been 

 measured in the same way as other forces, in terms of the 

 weight exercised by a given mass under the action of 

 gravity. The determination of the earth's magnetic force 

 requires two principal sets of observations. In the first 

 the earth's action upon a magnetic needle is (ippnsed to the 

 strength of a bar magnet. We iind in this way how many 

 times stronger is the action of the magnet on the needle 

 than is the earth's action. If we call H the force exerted 

 by the earth (so much of it, at least, as comes into play in a 

 horizontal direction) and M the strength of the magnet, then 



M 

 our experiment determines the numerical value of tt- In 



the second series of observations, we allow the earth's 

 magnetism to act with and reinforce the strength of the 

 magnet, and we determine the value of the two forces 

 acting together and reinforcing one another. Our 

 numerical result is now in the form of a proiluct, viz., the 

 strength of magnet multiplied by the horizontal component 

 of the earth's magnetic force, or M x H. The method 

 employed is to suspend the same bar magnet as that used 

 in the last experiment by a fine wire or thread. The 

 magnet is placed in a stirrup, and the axis is horizontal. 

 Matters are arranged so that when the fibre is without 

 torsion the magnet lies in the magnetic meridian. The 

 magnet is then twisted round its point of suspension, 

 twisting the wire at the same time. Letting go the magnet, 

 we observe the rate of oscillation as the fibre twists and 

 untwists. The rate of oscillation is quicker than that of 

 an luimagnetized bar under the same conditions, for the 

 magnetic force acting between the earth and the poles of 

 the magnet assists the elasticity of the wire to overcome 

 the inertia of the iron bar. This acceleration of the 

 oscillation gives the value of M x H. In the former 



experiment we determined yt. The value of H can now be 



calculated, since if we divide the value M H (obtained 



when earth and magnet force act together) by „ (obtained 



when the earth acts against the magnet) the quotient is 

 the square of H. As we know the angle which the line of 

 action of the earth's force makes with the horizon (from 

 the (lip observation), we can readily calculate the total 

 magnetic force of which H is the horizontal component. 

 In some observations made in the north of England the dip 

 was 67^°. Hence the total force would be about twice the 

 horizontal component. In experiments in the same locality 

 of which we have the data before us it was found that H was 

 •1.5 dynes ; therefore, the earth's magnetic force was about 

 •3 dynes. The dyne is the unit of force of the centimetre- 

 gi-amme-second system of units, and it is equal to about -016 

 of the weight of a grain in the latitude of London ; -3 dynes 



is, therefore, equal to about one two-hundredth of a grain, 

 and -IS dynes, the value of H, to about one four-hundredth 

 of a grain. This small force, of the origin of which we 

 are ignorant, keeps the needle true to the pole. 



The rumour that new veins of very fine gold in abundant 

 working quantities have been discovered in British Guiana 

 will excite interest beyond City circles. The fact is that 

 the mineral resources of that land are unknown, and 

 although much of its natural history has been explained, 

 and obscure points partially cleared up, particularly of late 

 years, the field for original research, especially in the study 

 of the habits of animals, is almost unlimited. 



THE FACE OF THE SKY FOR APRIL. 



By Herbert Sadler, P.R.A.S. 



SUNSPOTS and facuhp show but small signs of 

 decreasing in number or size. Conveniently 

 observable minima of Algol occur at llh. 22m. 

 P.M. on the 8th, and 8h. 10m. p.m. on the 11th. A 

 maximum of the beautiful variable star E Leonis 

 takes place on the 11th. 



Mercury is too near the Sun to be observed with any 

 advantage in April. 



Venus is an evening star, and is well situated for 

 observation. On the 1st she sets at 9h. 17m. p.m., or 2|h. 

 after the Sun, with a northern declination of 15° 9', and 

 an apparent diameter of llf", toV*'^^ of the disc being 

 illuminated. On the 11th she sets at 9h. 50m. p.m., with 

 a northern decUnatiou of 19° 0', and an apparent diameter 

 of 12i", tVo'''i3 of the disc being illuminated. On the 

 21st she sets at lOh. 19m. p.m., or 3Jh. after the Sun, 

 with a northern declination of 22° 14', and an apparent 

 diameter of 12|", /(fgths of the disc being illuminated. 

 On the 30th she sets at lOh. 44m. p.m., with a northern 

 declination of 24° 13', and an apparent diameter of ?.3j", 

 y'J^ths of the disc being illuminated. During the month 

 she passes through a portion of Aries into Taurus. 

 Mars is, for the purposes of the observer, invisible. 

 Jupiter is an evening star, and is still well situated 

 for observation. On the 1st he sets at Ih. 29m. a.m., with 

 a northern declination of 23° 27', and an apparent equa- 

 torial diameter of 36-| ", the phase on the f limb amounting 

 to 0'33". On the 12th he sets at Oh. 58m. a.m., with a 

 northern declination of 23° 29', and an apparent equa- 

 torial diameter of 35^". On the 22nd he sets at Oh. 25m. 

 A.M., with a northern decUnation of 23° 29', and an 

 apparent equatorial diameter of 34f ". On the 30th he sets 

 at llh. 56m. p.m., with a northern declination of 23° 28', 

 and an apparent equatorial diameter of 34-0", the phase 

 amounting to -i-jths of a second. He describes a direct path 

 in Gemini, from the north-west of r; Geminorum to the 

 north-west of jw, Geminorum. The following phenomena of 

 the satelUtes occur while the planet is more than 8° above 

 and the Sun 8° below the horizon. On the 1st a transit 

 egress of the shadow of the second satellite at 8h. 11m. 

 p.m. On the 2nd a transit ingress of the shadow of the 

 fourth satellite at lOh. 29m. p.m. On the 3rd an eclipse 

 reappearance of the third satellite at 8h. 4m. 58a. p.m. 

 On the 6th an occultation disappearance of the first satellite 

 at Oh. 9m. a.m. ; a transit ingress of the first satellite at 

 9h. 21m. P.M., of its shadow at lOh. 36m. p.m. ; an 

 occultation disappearance of the second satelhte at lOh. 47m. 

 P.M. ; a transit egress of the first satellite at llh. 38m. p.m. 

 On the 7th an eclipse reappearance of the first satellite at 

 lOh. Bm. 50s. p.m. On the 8th a transit ingress of the 

 shadow of the second satellite at 8h. 6m. p.m., a transit 



