256 



NATURE 



[July 29, 1875 



First, " The electromotive force of a voltaic circuit varies 

 Avith the number of the elements, and the nature of the 

 metals and liquids which constitute each element, but is 

 in no degree dependent on the dimensions of any of their 

 parts." Second, " The resistance of each element is directly- 

 proportional to the distances of the plates from each other 

 in the liquid, and to the specific resistance of the hquid ; 

 and is also inversely proportional to the surface of the 

 plates in contact with the liquids." Third, " The resistance 

 of the connecting wire of the circuit is directly proportional 

 to its length and to its specific resistance, and mversely 

 proportional to its section." Some of the more important 

 forms of battery in use will now be described. 



Daniell's Battery, Fig. 42, consists of an earthenware 

 or glass vessel, within which a smaller jar of some porous 

 material is placed ; the space between the inner and outer 

 jars is filled with a dilute solution of sulphuric acid and 

 water, and within the porous jar a saturated solution of 

 sulphate of copper ; a cylinder of zinc is immersed in the 

 acid solution, and a cylinder of copper in the sulphate 

 solution, crystals of sulphate of copper being introduced 

 to maintain the strength of the copper medium. The 

 current from this battery is remarkably constant, a matter 

 of the greatest importance in the working of a telegraphic 

 circuit, as with a variation in the working strength of the 

 current, continued adjustment of the transmitting and 

 recording apparatus is rendered necessary. Bunsen's 

 battery (Fig. 43) in many respects resembles the Daniell 

 arrangement ; carbon is used within the porous cell in 

 place of the copper cylinder, and nitric acid replaces the 

 saturated solution of sulphate of copper. The current 

 produced is stronger, but less constant than that from the 

 Daniell's cell. 



Many other arrangements for the generation of a voltaic 

 current for telegraphic purposes are in use, such as the 

 " Mari^ Davy " (Fig. 44), the " Leclanch^," and " Cal- 

 laud" batteries ; more or less, each has its special merits 

 and demerits : practically, the " Daniell " remains unsur- 

 passed. The essential condition of every practical form 

 of battery is that it shall produce a constant current be free 

 from local action, and possess mechanical facility of reno- 

 vation, with simplicity and economy of construction. 



7?. 



J_J^ 



Fig. 44.— The Msric Davj' sulphate of mercury Battery. 



The measurement of the value of every telegraphic 

 line as regards electrical resistance, as compared with 

 some ascertained standard of resistance, is a matter of 

 vast importance. By this means the electrical insulation 

 of a submarine cable or a land wire is definitely ascer- 

 tained, and the existence of a fault, together with its 

 locality, defined. Without some established utiit of re- 

 sistance by which to compare the working circuit with its 

 electrical equivalent, no test of insulation can be main- 

 tained or restoration of a circuit carried out. By general 

 acceptance a standard of measurement has been adopted, 

 a unit of resistance known as the B.A. (British Associa- 

 tion) unit. It is unnecessary to enter into detail as to the 

 mechanical problems which determine this unit of resist- 

 ance ; it is sufficient to state that the electrical resistance 

 or value of every circuit, land wire and submarine cable, 

 is now by universal acceptance recorded in B.A. units. 

 For instance, a guttapercha submarine cable core may 

 be stated to be so many hundred mi]lions B.A. units of 

 insulation test ; while, again, an indiarubber core may be 

 stated to be so many thousand millions of B.A. units of 

 resistance ; a correct comparison is thus at once deter, 

 mined. 



{To be continued.) 



OUR ASTRONOMICAL COLUMN 

 The Transit of Venus, 1882 December 6.— The 

 Greenwich time of first internal contact in this transit at 

 any point in these islands, according to Leverrier's Tables 

 of Sun and Planet, may be accurately found by the 

 following equation, in which / is the geocentric latitude 

 of the place, p the corresponding radius of the earth, and 

 L the longitude, reckoned positive, if east of Greenwich, 

 and negative, if west : — 



G.M.T. first Int. Cont. = Dec. 6d. 2h, i6m. i6s. 

 + [2'585S] p sin / - [2*4774] 9 cos /cos (l — 85° 58' -6) 

 The quantities within square brackets are logarithms ; 

 the correction of course results in seconds. Direct com- 

 putations for Greenwich, Edinburgh, and Dublin, furnish 

 the following particulars of the first internal contact at 

 these places :— 



Angle from 

 N. point. 



,. i°5o 40 . 

 . 150 42 . 

 . 150 41 . 



Angle from Sun's 

 Vertex. Altitude. 



. 128 2 ... 9"2 



. 132 8 ... 6-5 

 . 132 24 ... 9-6 



Local Time, 

 d. h. m. s. 

 Greenwich ... Dec. 6 2 21 2 

 Edinburgh... „ 2 8 46 

 Dublin ... „ I 56 8 

 At Greenwich the sun sets just one hour and a half after 

 Venus has wholly entered upon the sun's disc. 



The Sun's Parallax. — M. Liais, Director of the Im- 

 perial Observatory of Rio de Janeiro, has intimated his 

 intention to make a serious attempt to determine this 

 important element from the very favourable opposition of 

 the planet Mars, which will occur early in September 

 1877, being encouraged thereto by the success which 

 attended his observations about the opposition of i860, 

 when his instrumental appliances were very inferior to 

 what they are likely to be in 1877, The planet arrives at 

 perihelion on the 21st of August in that year, and in oppo- 

 sition at midnight on the 5th of September ; it is in 

 perigee on September 2nd at a distance of only 0*3767, 

 which is not far from the minimum, though slightly 

 greater than in the last three repetitions of the 79-year 

 period, as will appear from the following comparison : — 



Opposition. Mars — Mean Anomaly. 



1640-64 - 0° 12' 



1719-65 + 2 31 



1798-66 + 5 15 



1877-68 + 7 58 



The horizontal parallax of Mars will attain a value 

 which, as M. Liais remarks, will be sensibly equal to that 

 of Venus, diminished by that of the sun. With firm in- 

 struments and experienced observers, it is very probable 

 that the amount of solar parallax may be determined by 

 differential observations of Mars at the opposition of 1877, 

 with a precision which may be comparable with that 

 resulting from observations of a single transit of Venus. 



A Third Comet in 1813 (?). — Bode, after mentioning 

 in his Miscellaneous Notices {Berl. Jahrl?. 181 8) that 

 Canon Stark of Augsburg had observed the first comet of 

 18 1 3 on the 19th of February, states that Stark had also 

 discovered on the same evening with a 3^- feet DoUond 

 telescope, a very small and exceedingly famt comet with- 

 out tail above the variable star Mira in Cetus, the position 

 of which, by comparison with the variable, he found to be 

 at 7h. 28m. 37s., m R.A. 31° 17' 23", and Decl. 1° 52' 9" S. 

 He saw the comet a second time on the 20th, and again 

 comparing it v/ith Mira, and another adjacent star, its 

 place at 7h. 32m. 13s. was in R.A. 33° 47' 3", and Decl. 

 5° 49' 7" S. Cloudy skies are said to have prevented 

 further observation. Bode remarks, with respect to this 

 comet, that it is strange that no other astronomer had 

 perceived it, " doch versichert Herr Stark," he adds, 

 "noch in seinemletzten Schreiben anmich,aufs Heiligste, 

 die Richtigkcit dieser Wahrnehmung." However sus- 

 picious this circumstance may have appeared, we know 

 that several of the comets of short period have been 

 revolving in such orbits for one or two centuries, visiting 

 these parts of space without doubt under favourable 



