470 



NATURE 



\^August 29, 1878 



works Committee : being an Account oj Excavations in Ccesar's 

 Camp, Folkestone, and also a paper on Excavations at Mount 

 Caburn, Lewes, Sussex. — On Mount Caburn a large number of 

 pits were dug. They were found to contain iron implements 

 and arms entirely of a late Celtic type, and pottery of three 

 descriptions. The fact of these being of a late Celtic type was 

 proved by the getting of the specimens after a considerable 

 amount of digging, and the finding of British tin coins, which 

 were extremely rare, and which were known to be of a late 

 Celtic type. Then excavations were made through the rampart 

 in order to ascertain by means of relics found on the old surface- 

 land what was the date of the first Roman camp. In the upper 

 rampart a quantity of British pottery was found, showing the 

 probability that it was erected at an early time. In the outer 

 rampart pottery of a superior quality was found, by which means 

 it appeared likely that the camp was first erected during the 

 early British period, and was occupied up to the late Celtic 

 period. Subsequently the adjoining camp of Ranscombe was 

 cut through, and Samian pottery was found in the interior, 

 showing that it had been occupied by the Romans; but in the 

 body of the work nothing but British pottery was found, which 

 proves that, though occupied by the Romans, it was not con- 

 structed by them. The Ranscombe Camp may, therefore, have 

 been utilised by the Romans, as suggested by Horsfield a 

 long time ago, during an attack upon Mount Caburn. As to 

 Caesar's Camp, it was ascertained that the whole of the relics 

 found in the body of the several ramparts were of the Nor- 

 man period, nothing Roman or earlier having been found 

 anywhere. 



Report of Committee, consisiing of Prof. Harkness and Mr. 

 William Jolly, on the Fossils of the North- West Highlands of Scot- 

 land, by Mr, W. Jolly, tl.M.I.S., Secretary. — Since thelast report 

 one of the most active members of the Committee, the -late Dr. 

 James Bryce, has perished in the prosecution of his favourite 

 science. -For several years the Committee have explered the 

 great .limestone band running from Durness and Loch Eribol to 

 Loch Kishorn, opposite Skye. The fossils discovered were 

 6btained almost entirely from the Durness limestone, i'ourteen 

 miles east of Cape Wrath, the only other place where they have 

 been obtained in the limestone being at Indinadamph, or Loch 

 Assynt,' in the west of Sutherland. The considerable collection of 

 fossils made was placed in Dr. Bryce's care, but, on his unex- 

 pected decease, his collection was found to 'be more or less 

 scattered and unmarked, and the Committee have as yet been 

 unable to discover the Durness fossils, although cardfiil search 

 afld inquiries have been made; they hope to be aMe to report 

 their discovery to the next meeting. 



Through the good offices of several friends, and particularly 

 the Rev. W. C. Grant, a considerable collection has been made 

 from Garveilan, near Cape Wrath, while in the quartzites 

 of Lough Eribol numerous worm or annelid borings have been 

 obtained, and Mr. Mackay has this year discovered hitherto 

 undescribed fossil remains in the same quartzite. It is proposed 

 to have these carefully examined, and to endeavour to ascertadn 

 their bearing on the general problem of the place and succession 

 of rocks of the North- West Highlands. 



The Committee consider it desirable they should be re- 

 appointed, with a grant at their disposal to prosecute their 

 search for fossils, and also to examine and report on those 

 already obtained. 



SECTION A. — Mathematical and Physical. 



A New Determination of the Number of Electrostatic Units in 

 the Electromagnetic Unit. (Telegram and letter to Sir William 

 Thomson from Prof. W. E. Ayrton.) 



The Red Sea, August 3, 1878 

 From Singapore I sent you the following telegram on July 13. 

 •' Kindly inform British Association that air- condenser mea- 

 sured magnetically and statically gives mean value ratio of tbefe 

 units (29-80) twenty-nine point eight nouglit ohms. ; Foucault's 

 velocity light." 



In the autumn of this year I propose communicating a full 

 account of this investigation to the Physical Society or the 

 Society of Telegraph Engineers, or otherwise as you may ad- 

 vise ; but I thought that as the British Association Committee 

 nad for so long busied itself with the determination of electrical 



units you might deem the result of this investigation of Mr. 

 Perry and myself worthy of a preliminary notice at the meeting 

 of this Association to be held this year at Dublin. Not being 

 sure that I should arrive in Europe in time to reach Dublin at 

 the commencement of the meeting, although I hope to be pre- 

 sent during the last three or four days, I took the liberty of send- 

 ing you the telegram quoted above. 



The result we have obtained for " z/ " is the more interesting 

 inasmuch as without any bias being given to any one of our expe- 

 riments, the mean value is identically the same as that obtained 

 by M. Foucault for the velocity of light, whereas all previous 

 determinations of the number of electrostatic units in an electro- 

 magnetic unit have led to results differing considerably from 

 Foucault's velocity. 



It appeared to Mr. Perry and myself that the method best 

 suited for the accurate determination of v consisted in measur- 

 ing the capacity of an air-condens-er (i), electromagnetically 

 by the swing of the needle of a ballistic galvanometer ; and (2) 

 electrostatically by a measurement of the linear dimensions of 

 the condenser, since the equation connecting these capacities 



s = v^S 

 s being tho absolute electrostatic capacity, 

 S ,, electromagnetic ,, 



leads to an equation involving only the square root of a 

 resistance. 



Two difficulties of course presented themselves in this inves- 

 tigation, difficulties that it took us many months to overcome, 

 labouring as we were under the disadvantage of experimenting 

 in a country like Japan. They were — 



(1) To obtain a large air- condenser of which the plates had 

 sufficiently true surfaces that the electrostatic capacity could be 

 accurately measured ; at any rate when the plates were not 

 nearer than half a centimetre to one another. 



(2) To obtain a galvanometric arrangement of sufficient sensi- 

 bility to measure the small capacity of such an air condenser 

 and sufficiently ballistic that the air damping should be almost 

 inappreciable. 



A full description of the condenser we employed (and which 

 had a guard ring, and all the different arrangements we could 

 think of for obtaining accurate results) will accompany the 

 account of the investigation to which I have referred. It is 

 sufficient here to mention that the .errors arising from the sur- 

 faces of the condenser plates not being true planes were prac- 

 tically eliminated by capacity experiments being made with 

 successive adjustments of the condenser plates, a difTerent set of 

 points in the upper plate being each time brought to the fixed 

 distance from the lower plate. 



The arrangement of a ballistic galvanometer to fulfil the two 

 conditions mentioned in (2) was very troublesome. I made 

 several astatic needles none of which satisfied us, and we were 

 beginning to fear my departure from Japan would necessitate 

 the abandoning of the investigation. At last, however, an 

 astatic combination containing forty small magnets (and of 

 which a -description will accompany the paper), gave satisfactory 

 results, and I obtained three excellent sets of observations on 

 June 18, June 23, and June 25, when my departure put an end 

 to further investigation. The w^aw values obtained for "z/" 

 on^ach of these three days were — 



29*74 ohms, June 18 

 29'95 'm .. 23 

 2972 „ „ 25 



Final mean 29 80 



It will be observed that the greatest difference between any 

 one of the three daily means and the final mean, is only about 

 half per cent. The final mean 298*0 million metres per second 

 (and which represents the mean of ninety-eight discharges of the 

 air-condenser) may, I think, be regarded as correct to, at any 

 rate one per cent, and is exactly equal to M. Foucault's velocity 

 of light. 



In the astatic combinations I employed jM-ior to June 18 I 

 used eight needles, and weighted the lower set of needles with 

 pieces of brass, so as to give it a barrel shape, but the results 

 were unsatisfactory, as there was either not sufficient delicacy or 

 else too much damping. Consequently all the numbers obtained 

 prior to June JS have been abandoned. On June 18 were 



