452 



NATURE 



\ April Z, 1875 



This is a very important fact to bear in mind, as by the 

 measurement of the copper resistance of the conductor in 

 a cable, a basis is at once established by which to deter- 

 mine the distance of a fracture. Knowing the value of 

 the resistance of the whole length of the cable conductor 

 — assume for 2,000 miles the value to be 2,000 units (the 

 measure of the unit being the resistance of one mile of 

 the copper conductor) — an interruption occurs, continuity 

 is broken, and the copper resistance only gives 760 and 

 1,240 units respectively when measured from either end. 

 Thus is clearly established a basis upon which the ap- 

 proximate distance of the "fault" may be ascertained. 

 Again, it was pointed out that the insulating medium sur- 

 rounding the conducting wire absorbed an appreciable 

 amount of electricity in the passage of the current through 



-Dt.comp( 



force) of the \ulLa c battcr> 



the conducting wire. This absorption may be taken as a 

 constant quantity, and the absorption for any length of 

 cable be determined from given data as regards the 

 time of electrification or the saturation of the circuit, and 

 the time of discharge, or the percentage of leakage from the 

 mechanical imperfections of all the insulating substances. 

 Thus again is established a process by which, under 

 certain conditions of injury to a ca.ble, by correctly mea- 

 suring the discharge, the position of a fault may wilh 

 more or less accuracy be localised. The commercial 

 value of a submarine cable depends upon the rapidity of 

 its transmitting capacity, and the speed depends upon the 

 time required to produce a variation in the tension of the 

 current at the distant end sufficient to influence the 

 recording instrument. The working speed depends, 

 therefore, upon the delicacy of the apparatus employed, 

 as then a small difference in the tension will suffice. In 

 cables similarly constructed, but of different length, the 

 speed of each is inversely proportional to the square of 

 the length ; because, when the length is doubled, 

 the capacity for charge is doubled, and the electrical 

 waves of charge and discharge have twice the dis- 

 tance to travel ; therefore the retardation is increased 

 fourfold. When the dimensions and weight of the insu- 

 lating medium are fixed, there is a loss of speed if the 

 conducting wire is too small ; and again, if the con- 

 ducting wire is too large, the speed is reduced by the 

 increased capacity of the wire in a greater degree than it 

 is augmented by the reduced resistance of the wire. The 

 best accepted ratio of the insulator to that of the conduc- 

 tor is when the insulator is somewhat less than 3.\ times 

 that of the copper conductor, or, more accurately speaking, 

 in the proportion of 3'4i of insulator to i of copper. On 

 long cables and where high speed is required, every 

 current transmitted through the cable should be at equal 

 intervals and of equal duration, so that the charge may 

 be maintained constant between the signals. 



(To be continued^ 



ECLIPSE OF THE SUN, APRIL 6 



AS no telegram has been received from Dr. Schuster's 

 party on its arrival at Singapore, we are compelled 

 to estimate the date of its arrival by the telegram in 

 yesterday's papers, which informed us that the Pern, in 

 which vessel the Expedition was conveyed from Galle, 

 arrived at Shanghai. The vessel was due there on the 

 3rd, and arrived on the 5th. Assuming all the delay to 

 have occurred on this side of Singapore, Dr. Schuster's 

 party would have reached that place on the 24th of March, 

 which would give them ample time to reach Chulai Point 

 and make their preparations, especially as the colonial 

 steamer which has been detached for the service is very 

 swift. 



It is not probable that news will be received from either 

 of the parties for some little time, as it will probably be 

 carried by local steamers to Rangoon, Singapore, or 

 Calcutta. 



In the meantime we take the following extracts from 

 an article in the Times of Tuesday, showing the final 

 arrangements adopted so far as they are known : — 



" The advantages of scientific, and especially of astro- 

 nomical expeditions, are by no means confined to the 

 record of those special phenomena which the observers 

 go out to see. The growing interest taken by all classes 

 in the study of nature, while it makes a large number 

 anxious to participate in the results obtained, at the same 

 time puts them in presence of a class of facts which the 

 stay-at-home student finds it hard to realise for himself. 

 The total eclipse of the sun, which is visible in the Nicobar 

 Islands, Burmah, Siam, and Anam to- day is a case in 

 point. While early risers are breakfasting this morning, 

 with the beams of the sun, low down in the east, not yet 

 able to break through the morning mists, some quarter 

 of the way round the world there will be at least three 

 parties of anxious observers battling with the fierce noon- 

 tide heat of that same luminary nearly overhead, soon, 

 indeed, to have his light and heat entirely withdrawn for a 

 time, but, all the same, under conditions so different from 

 those we are familiar with here, that the sun and the sur- 

 roundings of the observers might seem to form part ot 

 another universe. Another point — and this is one which 

 will doubtless disappoint many — is that this eclipse, 

 which, as we stated in a former article, on the high autho- 

 rity of Mr. Hind, in the time of obscuration will not be 

 surpassed by any other available one during the present 

 century, is totally invisible here. Although tfiere is almost 

 total darkness for nearly five minutes in Burmah and 

 Siam, no trace of an eclipse will be seen in these islands, 

 for the reason that although it began as early as two 

 minutes to four this morning, and continued till sixteen 

 minutes past nine, the moon's shadow falls first to the 

 south, and then to the east of us. In fact, the line of 

 total eclipse runs from the Cape of Good Hope to Burmah 

 and Siam, and thence to the North Pacific. We lie, 

 therefore, in no part of the track of the shadow. 



'• To pass from what may be considered geographical 

 considerations, we may remind our readers that in a 

 former article (the Times, Jan. 11, reprinted in NATURE, 

 p. 201) we pointed out the value which many men of 

 science attached to securing observations of this eclipse, 

 and we attempted to give a general statement of the various 

 questions pressing lor solution, which, in the opinion 

 of the Council of the Royal Society, justified an appli- 

 cation to the Government for aid, not only in sending 

 out expeditions from this country, but in organising 

 a party of observers in India. Our readers have also 

 been informed (the Times, Jan. 16) of the fact that 

 the application to Government was at once acceded to 

 in the warmest manner, and that Sir Stafford North- 

 cote, the Marquis of Salisbury, and the Viceroy of India, 

 as well as the Admiralty authorities, have been un- 

 ceasing in the encouragement and assistance which they 



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