March 14, 1889] 



NATURE 



471 



the very heavy rains. The highest mountain-range of the whole 

 region is a little s )Uth of Chiengtung, and is about 4400 feet 

 high. After leaving Miiang Hai, no signs of habitation were 

 seen until Chiengtung was reached. As an instance of the in- 

 security of the country, he mentions that with one party he saw 

 a young woman armed like the men. The plateau of Chieng- 

 tung is about 27^,0 feet absve the sea-Ievei, and is very bare and 

 badly cultivated. The town itself is walled, and is situated in 

 the southern corner of the plateau, which is inclosed by high 

 mountains on the west, south, and cast. The chief products are 

 rice, cotton, and opium ; tea is also cultivated on the hills. 

 Cotton goods are imported from Moulmein ; salt, silk, and other 

 articles from Yunnan ; cocoa-nuts and betel-nuts from Chienghai. 

 The inhal)itants of the district belong to several races ; the 

 majority are of the same people as the ruling family, but there are 

 also numbers of Western Shans, of the hill tribes (the Kins, Kans, 

 and Musos), and Lems and Liis. The houses of the people are 

 as miserable as could be, and their great poverty is shown hy the 

 fact that, instead of their temples being made of brick, as is the 

 <;ase in Siam, they are wretched i.heds, ahnost as miserable as 

 the houses. Mr. Archer returned by a different road, and found 

 whole districts uninhabited between Chiengtung and Muang Len, 

 which is close to the Cambodia River. 



The Manche ter Guardian publishes some interesting informa- 

 tion as to the movements of Dr. Lansdell. In October last he 

 arrived at Khoten, whence he set out for Yarkaud. He left 

 Yarkand for Western Tibet and crossed the Kilian Pass at a 

 height of 1 7,cxx) feet on December 2. On the loth the Kara- 

 koram was passed without very much difficulty ; but a few days 

 later, when crossing the Saser, the party suffered very much 

 from the intense cold. On November 14, Dr. Lansdell sighted 

 at ChangJung the first inhabited huts of the Tibetans. Crossing 

 over Khardung Pass with great difficulty, he finally arrived at 

 Leh, where he remained for a few days, setting out on December 

 5 to descend to Kashmir. To aid in this attempt, forty men 

 were sent by the local authorities, and Zogiha Pass was crossed 

 in safety, and Kashmir reached. In a few days Srinagur was 

 reached, but Soultem Passes were blocked, and an attempt to 

 penetrate them was abandoned. At Kawul I'indee, Dr. Lans- 

 <leirs journey finished, he having travelled from Kuldja 2000 

 miles, crossing seven of the highest passes in the world. 



The Government Geologist who set out from Adelaide some 

 months ago to explore Central Australia has just returned. He 

 travelled as far north as Alice Springs — that is, to the Tropic of 

 Capricorn— and spent nearly a fortnight there examining the 

 ruby and gold fields. Between Anna Creek and Alice Springs 

 the countiy is well watered, but at the diggings water is very 

 scarce. In all, some 2400 miles were travelled, and, around the 

 ruby fields, camels were employed. The heat was very great, 

 on one occasion reaching as much as 124" F. in the shade. 



THE DISCHARGE OF A LEVDENJAR.^ 



T T is one of the great generalizations established by Faraday, 

 that all electrical charge and discharge is essentially the 

 charge and discharge of a Leyden jar. It is impossible to charge 

 one body alone. Whenever a body is charged positively, some 

 other bofly is ipso facto charged negatively, and the two equal oppo- 

 .site charges are coipnected by lines of induction. The charges 

 are, in fact, simply the ends of these lines, and it is as impos- 

 sible to have one charge without its correlative as it is to 

 have one end of a piece of string without there being some- 

 where, hidden it may be, split up into strands it may be, but 

 somewhere existent, the other end of that string. 



This I suppose familiar fact that all charge is virtually that 

 of a Leyden jar being premised, our subject for this evening is 

 at once setn to be a very wide one, ranging in fact over the 

 whole domain of electricity. For the charge of a Leyden jar in- 

 cludes virtually the domain of electrostatics ; while the discharge 

 of a jar, since it constitutes a current, covers the ground of 

 current electricity all except that portion which deals with phe- 

 nomena peculiar to steady currents. And since a current of 

 electrichy necessarily magnetizes the space around it, whether it 

 flow in a straight or in a curved path, whether it flow through 

 wire or burst through air, the territory of magnetism is likewise 

 invaded ; and inasmuch as a Leyden jar discharge is oscillatory, 



March 



Friday evening discourse at the Royal In.sthution of Great Briuin, on 

 ■ - by Pfuf. Ol.ver J. L dg^, F.R.S. 



and we now know the vibratory motion called light to be really 

 an oscillating electric current, the domain of optics is seriously 

 encroached upon. 



But though the subject I have chosen would permit this wide 

 range, and though it is highly desirable to keep before our minds 

 the wide-reaching import of the most simple-seeming fact in 

 connection with such a subject, yet to-night I do not intend to 

 avail myself of any such latitude, but to keep as closely and dis- 

 tinctly as possible to the Leyden jar in its homely and well- 

 known form, as constructed out of a glass bottle, two sheets of 

 tinfoil, and some stickphast. 



The act of charging such a jar I have permitted myself now 

 for some time to illustrate by the mechanical analogy of an in- 

 extensible endless cord able to circulate over pulleys, and thread- 

 ing in some portion of its length a row of tightly-gripping beads 

 which are connected to fixed beams by elastic threads. 



Mechanical anaio^y of a circuit partly dielectric : for instance, of a cli.irged 

 condenser, a is its positive coat, n its negative. 



The cord is to represent electricity ; the beads represent suc- 

 cessive strata in the thickness of the glass of the jar, or, if you 

 like, atoms of dielectric or insulating matter. Extra tension in 

 the cord represents negative potential, while a less tension (the 

 nearest analogue to pressure adapted to the circumstances) 

 represents positive potential. Forces applied to move the cord, 

 such as winches or weights, are electromotive forces ; a clamp 

 or fixed obstruction represents a rheostat or contact-breaker ; 

 and an excess or defect of cord between two strata of matter 

 represents a positive or a negative charge. 



The act of charging a jar is now quite easily depicted as shown 

 in the diagram. 



To discharge the jar one must remove the charging E.M.F. 

 and unclamp the screw, i.e. close the circuit. The stress in the 

 elastic threads will then rapidly drive the cord back, the inertia 

 of the beads will cause it to overshoot the mark, and for an 

 instant the jar will possess an inverse charge. Back again the 

 cord swings, however, and a charge of same sign as at first, hut 

 of rather less magnitude, would be found in the jar if the opera- 

 tion were now .suspended. If it be allowed to go on, the 

 oscillations gradually subside, and in a short time everything is 

 quiescent, and the jar is completely discharged. 



All this occurs in the Leyden jar, and the whole series of 

 oscillations, accompanied by periodic reversal and re-rever>al of 

 the charges of the jar, is all accomplished in the incredibly i^hort 

 space of time occupied by a spark. 



. Consider now what the rate of oscillation depends on. Mani- 

 festly on the elasticity of the threads and on the inertia of the 

 matter which is moved. Take the simplest mechanical analogy, 

 that of the vibration of a loaded spring, like the reeds in a 

 musical box. The stiffer the spring, and the less the load, the 

 faster it vibrates. Give a mathematician these data, and he will 

 calculate f)r you the time the spring takes to execute one 

 complete vibration, the "period" of its swing. [Loaded lath 

 in vice.] 



The electrical problem and the electrical solution are precisely 

 the same. That which corresponds to the flexibility of the 

 spring is in electrical language called static capacity, or, by 

 Mr. Heaviside, permittance. That which corresponds to the 

 inertia of ordinary matter is called electro-magnetic inertia, or 

 self-inductiop, or, by M". Heaviside, inductance. 



