August 30, 1900] 



NA TURE 



A. 



413 



formed a plaster on the walls, its thickness being at most three 

 inches. It showed prismatic structure, though rather small. 

 The air within was cold (I had no thermometer) ; but as the 

 surface of the ice was wet, it was above 32" P\, though I think 

 not much. The guide told me that the fissures in winter-time 

 were filled with snow. This accumulation, probably owing to 

 the shape of the fissure, no longer remained as snow, but was 

 represented by the ice on the floor and walls, which the guide 

 said seldom, if ever, disappeared. The absence of ice from the 

 walls of the Ingleborough" swallow hole" was probably due to 

 some exceptional dryness of the rock ; but Prof. Hughes has un- 

 doubtedly found a "baby" ice-cave, like that I have described, 

 and it will be worth examining some more of these dry shafts to 

 see whether a slightly better developed specimen may not be 

 lurking in the neighbourhood. T. G. BONNEY. 



Permeability of Iron under the influence of the 

 Oscillatory Discharge from a Condenser. 



In your issue of August 2 there is an abstract of a very interest- 

 ing paper, read by Prof. Trowbridge, on his experiments with a 

 battery of 20,000 secondary cells. In it he mentions that the 

 permeability of iron when under the influence of a very powerful 

 discharge from a large condenser is now under observation. 



I should like to draw attention to some experiments I was 

 making over a year ago in Lord Blythswood's laboratory (an 

 account of which has not yet been published), in which I have 

 gone into the subject in some detail. 



In my experiments the lowest frequency used was about 

 5000 a second. I enclose two photographs of sparks taken in 



the usual way with a revolving mirror. The discharge in photo- 

 graph (i) took place through a coil of about 5 millihenrys self- 

 induction from a battery of Leyden jars of a total capacity of 

 •06 microfarads, the potential difil'erence between the coatings, 

 before discharge, being 13,500 volts. In photograph (2) a fine 

 wire core, consisting of 550 No. 28 soft iron wires, was inserted 



into the coil (which was wound on a hollow paper spindle of 

 about I '3 cm. internal diameter). The other conditions of 

 discharge were identical in the two cases ; the speed of the 

 mirror, however, was 19 revolutions per second for photograph 

 (i), and 16 per second for photograph (2), thus tending to draw 

 out the spark more in the first photograph. 



The essential differences are, however, well marked. At the 

 beginning of the discharge we have the "pilot" spark, first 

 noticed by Prof. Boys ; and then in the photograph (2), taken 

 with the iron w're cores, a series of oscillations gradually in- 

 creasing in length. The first half-oscillation, however, is nearly 

 twice as long as the half-oscillations in photograph (i), when 

 there were no iron wire cores in the coil. The increase in the 

 time for a half-oscillation is due, of course, to the increased self- 

 induction of the coil on account of the iron; and the gradually- 

 increasing length is due to the increase in permeability of the 

 iron as the intensity of the discharge dies away. In photograph 

 (i) the frequency of oscillation of the spark taken with the coil 

 having air cores is about 9000 per second, and in (2) the ap- 

 proximate magnitude of the current during the first discharge 

 with the iron cores, 15 amperes. 



It would be impossible in the course of a short note to de- 

 scribe in detail the work that has been done, but in numerous 

 experiments (over three hundred spark photographs have been 



NO. 1609, VOL. 62] 



taken) that have been made, the iron has been found to behave 

 in the same way under these oscillating magnetisations as it does 

 when steady currents are used to produce magnetising forces of 

 the same intensity. In most experiments single layer coils have- 

 been used in which the magnetising forces due to a given cur- 

 rent can be calculated, and it has been possible to determine 

 approximately the forces acting on the iron. From the results, 

 curves showing the variation in permeability with magnetising 

 force have been plotted. In some experiments, the magnetising 

 current due to the discharge has been as large as 1000 amperes. 

 In order to obtain discharges as powerful as this, a very large 

 glass-condenser has been used with a total capacity of 15 

 microfarads, made up of plates of glass (coated with shellac) 

 I '6 mm. (1/16") thick. The conducting surfaces are of tinfoil. 

 The glass appears a great deal stronger than that used by Prof. 

 Trowbridge, as it has been tested repeatedly at 20,000 volts. It 

 is possible, however, that the suddenness with which his con- 

 denser is charged from his cells may account for the readiness 

 with which the glass breaks. In my experiments the condenser 

 was charged by a large Wimshurst machine of 160 plates, which 

 took almost half a minute to get up the full potential of 20,000 

 volts. The glass used is known technically as 15 oz. 3rds 

 selected flat sheet, and was obtained from Messrs. Malloch, of 

 Glasgow. E. W. Marchant. 



Blythswood Laboratory, Renfrew, August 7. 



Function of the Whips of the Larva of the 

 Puss Moth. 



Y.OUR correspondent (p. 389) will find a detailed account of 

 the various defensive appliances of the larva of Centra vinula 

 in Prof. Poulton's work on the "Colours of Animals " (Inter- 

 national Science Series), and in papers published by him in the 

 Transactions of the Entomological Society of London for 1886 

 and 1887, the latter papers being illustrated by beautiful coloured 

 plates. 



It is usually believed by entomologists that the function of 

 the " whips" in the caudal appendages of the larva is to drive 

 away, or frighten away. Ichneumon Flies or other enemies, but 

 there is still room for further inquiry ; and although the larva is 

 highly protected, it is liable to the attacks of .some species of 

 Ichneumon Flies, though it may be able to defend itself against 

 others, for the protection of no animal is absolutely complete. 



The appendages are doubtless homologous with the retractile 

 fleshy fork in the neck of the larvoe of the Swallow-tailed 

 Butterflies (Papilionidae), which probably fulfils a similar 

 function. W, F. Kirby. 



Hilden, Sutton Court Road, Chiswick. 



The Migration of Swifts. 



On the morning of Friday, August 10, I witnessed a large 

 flight of Swifts travelling westward along the Sussex coast. The 

 birds were passing this place in a continuous though thin stream 

 for several hours ; I saw them myself from 10 a.m. when I first 

 visited the shore, and watched them till 12 noon. A few birds 

 were also noted travelling in the same direction between 5 p.m. 

 and 6 p.m. The day was bright but showery, and a fresh 

 W.N. W. breeze was blowing at the time, so that the birds were 

 flying almost against the wind ; they flew low, seldom rising 

 fifteen feet in the air, and often passing within two feet of my 

 head as I lay on the shingle ; they kept to the coast-line and 

 for the most part over the top of the fringe of tamarisks that 

 here stretch for miles just above the shingle. Since that day I 

 have not seen a single Swift in the neighbourhood, in spite of 

 having travelled on my bicycle as far west as the mouth of 

 Chichester Harbour along the coast, and to various places north 

 of this line as far as Chichester and Arundel inland. It would 

 be interesting to know if other observers witnessed any similar 

 flights on August 10, and also if Swifts are still to be seen in any 

 places in our islands at the present time. I have on two previous 

 occasions seen Swifts arrive on the east coast of Norfolk as late 

 as the first week in September (after a complete dearth of the 

 birds for some three weeks), and depart again after a few days' 

 sojourn — these perhaps are migrants from the European conti- 

 nent. As many of your readers are now doubtless at the seaside, 

 it seems a favourable opportunity to ask them to keep their eyes 

 open and record any facts that they may observe bearing on the 

 movements of the.se birds. Oswald H. Latter. 



East Preston, near Worthing, August 19. 



