November 4, 1909J 



NA TURE 



High Pressure Spaik Gap in an Inert Gas. 



For some years I have employed a high-pressure spark- 

 gap, such as that described by me in the Fhil. Mag. for 

 August, 1902, in connection with a Testa inductive system, 

 and 1 have experienced considerable trouble arising from 

 the erosion of the spark balls and their supports. They 

 soon become coated with an oxide of the metal employed, 

 and the sparking becomes unsteady. As a cure for this 

 evil, which contributed much to the formation of a bad 

 type of spark, the employment of some inert gas suggested 

 itself to me ; and of such gases Mr. C. C. F. Monckton 

 proposed the use of nitrogen, and this gas I now use instead 

 of air. I find that after the continuous use of nitrogen 

 in the spark-gap the balls are but little affected, while the 

 spark through a gas pressure of 50 lb. per square inch 

 is compact and constant in shape, and the yield of the 

 induction apparatus is greatly enhanced. The spark-gap 

 globe is filled to the required pressure from the gas cylinder 

 through a reducing valve, and when it is shut off the 

 pressure is maintained for ten or twelve days nearly up to 

 the initial one. 



The nitrogen, which was supplied by the British 

 Oxygen Co., compressed in a steel cylinder, turned out 

 to be very nearly pure ; it is separated from liquefied 

 air, and is certajply more pure than hydrogen as sup- 

 plied in cylinders, and gives better results. The spheres 

 are made of thick white glass, and are tested to 

 about four times the load they are worked under. The 

 spark ball is advanced by means of a fine screw forty- 

 eight threads to the inch, cut on ^-inch rod, working in a 

 boss which forms a part of the gun-metal cap with which 

 the glass globe is closed. If the screw is carefully fitted 

 by Whitworth screwing apparatus, no gland or stuffing- 

 box is required. The screw is slightly lubricated with a 

 mixture of equal parts of pure india-rubber and vaseline. 

 The length of the spark is measured by means of a divided 

 head attached to the screws 



It might be supposed that a long spark in air at normal 

 pressure would have the same effect as a spark shortened 

 by gas pressure ; but experimental evidence shows that the 

 thick, steady, well-formed spark made under pressure gives 

 far the most trustworthy results. Sparks made in air at 

 normal pressure often do not strike from the nearest 

 surfaces, but strike along an arched path, this effect 

 reducing the discharge and rendering it variable in its 

 intensity ; but when the high-presseure nitrogen spark-gap 

 is employed, the discharge from the Tesla apparatus is 

 steady and unvarying during periods of time such as forty 

 or sixty minutes. F. J. Jervis-Smith. 



The Small Motion at the Nodes of a Vibrating String. 



It is generally recognised that the nodes of a string 

 which is maintained permanently in oscillation in two or 

 more loops cannot be points of absolute rest, as the energy 

 requisite for the maintenance of the vibrations is trans- 

 mitted through these points. I have not, however, seen 

 anywhere a discussion or experimental demonstration of 

 some peculiar properties of this small motion. A brief 

 note may therefore be of interest. 



In the first place, the small motion at the node is in 

 a phase which is different from that of the rest of the 

 string. The exact difference of phase is shown by a 

 dynamical investigation to be a quarter of an oscillation. 

 The motion is of very small amplitude, and it might there- 

 fore be thought a difficult matter to verify this experi- 

 mentally. I have, however, devised some convenient 

 arrangements with which this can be effected. I shall here 

 mention only one method : this was to compound the 

 oscillation at every point on the string with another per- 

 pendicular to it of half the frequencv, and to observe the 

 compound oscillation at the nodes and elsewhere. 



Such a compound oscillation can easily be maintained 

 permanently by having the string attached to the prong 

 of an electrically maintained tuning-fork, so that it lies 

 in a plane perpendicular to the prongs, but in a direction 

 inclined to the line of their vibration. When the load on 

 the string is slightly greater than that necessary for the 

 most vigorous maintenance, points on the string describe 

 parabolic arcs with concavities in opposite directions in 

 NO. 2088, VOL. 82] 



alternate loops, the whole forming a beautiful and interest- 

 ing type of stationary vibration. This is not, however, 

 the stage convenient for observing the small motion at 

 the nodes. When the tension of the string is relaxed, so 

 as to make its vibration stronger, points on the string, 

 i.e. except the node, describe S curves. The curve 

 described by the node is neither a straight line nor an 

 S curve, but is a very flat parabola. From this, the phase- 

 relation between the small motion at the nodes and the 

 large motion elsewhere is obvious. 



If the node has a small motion, then, strictly speaking, 

 there is no node at all. There should, however, be points 

 at which the positions of the string in opposite phases 

 might be supposed to intersect. One might suppose that 

 these points, or " fictitious nodes," should execute a very 

 small, almost microscopic, niovement. .^s a matter of 

 fact, these " fictitious nodes " oscillate parallel to the 

 string through a range equal to the whole length of a loop. 

 This somewhat striking effect may be observed without 

 difficulty by illuminating the string with periodic illumina- 

 tion of twice the frequency of the oscillation. 



Post-Box 59 Rangoon. C. V. Ram.in. 



An Instance of Prolonged Pupation. 



The following facts in connection with a specimen of 

 the privet hawk-moth may possibly be of interest to 

 some of your readers. 



The caterpillar, which was reared from the egg at Tun- 

 bridge Wells, pupated between August 7 and g, 1906, 

 and the pupa was sent out to me by post. The moth did 

 not emerge until October 16, 1908, having been more than 

 two years in the pupal state. Being the only specimen I 

 have, I cannot say whether it shows any variations ; but 

 it is not stunted, measuring just over 4 inches across the 

 expanded wings. Geo. H. Wyld. 



Svdnev, N.S.W. 



.4 SCIENTIFIC MISSION IN ETHIOPIA.' 

 A BYSSINL'V — and even in a more general way the 

 ■'*- whole Empire of Ethiopia — though it was the 

 first portion of tropical Africa to come within what 

 one might term the scientific cognisance of the 

 civilised world, the world of Mediterranean Europe 

 and Western .Asia, remains, nevertheless, to this day 

 the least understood and one of the most imperfectly 

 explored parts of .Vfrica. In all probability, more is 

 known about the fauna, the flora, the human races, 

 and languages of even the most recently revealed 

 recesses of the Congo Basin, of the Central Sudan, 

 the Liberian hinterland, and the south-western part 

 of the \iger Basin (to mention some of the least- 

 known parts of tropical Africa) than is recorded of 

 the dominions of the Emperor Menelik. 



This ignorance of Ethiopia (from the point of view 

 of modern science) is, of course, proportionately 

 estimated in relation to the extraordinarily important 

 position all this region occupies in the study of 

 .Africa, in the solution of .African enigmas. It is an 

 area of about 200,000 square miles, containing excep- 

 tionally high mountains, the tops of which, but for the 

 increasing aridity of North-East .Africa, should be 

 even more covered with glaciers than is the case with 

 Ruwenzori, under the Equator, for the Ruwenzori 

 range only exceeds in altitude by a few hundred feet 

 the estimates of the highest points of northern and 

 south-western .Abyssinia. In Abyssinia alone, of all 

 parts of tropical or Trans-Saharan .Africa, may dis- 

 tinct evidences be found of the existence (on the high 

 mountains and even in the plains) of a Eurasian fauna 

 and flora — wild swine of the genus Sus, wild goats, 

 wild dogs {Caiiis simiensis), and a few other beasts 



1 "Mission en Ethiopie (igoi-3)." By Jean Duchesne-Fournet and 

 others. Tome i,, pp. xviii + 440; Tome ii., pp. xv-t-388, and atlas. (Paris: 

 Masson et Cie. , 1909.) 



