Ft£. 10, 1882.] 



KNOWLEDGE 



313 



words, the outline of the planet will be seen to differ 

 soiisibly from that of a circle, a portion of such outlme or 

 " limb " being seemingly cut oft' by a curved line on the 

 side opposite the Sun. This gives a somewhat hump- 

 backed effect. (Lat (.ibbiis, humped.) Hence the 



o 



Fig. 17.— Mais 



While in this region of the heavens, we may notice one 

 01 two objects in Taurus, which we omitted on p. 221, in 

 the absence of means for tlieir identification. They will 

 be found in the Zodiacal Map, which we are now em- 

 ploying. The first is Piazzi ^^, 20 Tauri, which will be 

 noticed in the second square to the left of, and a little 

 higher than Aldebaran ; it is marked 20". This will 

 prove a very severe test of the excellence of the observer's 

 in.strument, and will require a fine night and the highest 

 power at his disposal to be made out properly. 118 Tauri, 

 again, is a beautiful small pair ; it lies on the parallel of 

 declination below /J on the map. In noticing the nebula 

 to the N.W. of ; Tauri, we omitted to add that 4 itself 

 is situated in a rather pretty and curious field. 



Above Taurus lies the constellation Auriga, to the 

 examination of which we proceed to devote ourselves. We 

 will begin with 14, marked T (for triple) in the map ; but wc 

 shall only be able to see it as a double star, the components 

 being of a yellowish tint, and about half as far again apart 

 as those of y Arietis. A very pretty pair will be found in 

 hi Aurigiv. This does not appear by name on the map, 

 but is so close to the star at the top marked 4° as to in- 

 cline us to believe that they are intended for the same 

 object. It is represented in Fig. 18. 



6 Aurig;e, a."* a close and very unequal pair, will ta.x both 

 the instrument and the eyesight of the observer to the 

 Uttermost to see it properly. When best seen it will ap- 

 pear as in Fig. 19. 



10.— 9 Auriga?. 



5. Aurigie (to the north of w, or 4, just out of the map, 

 on p. 225), is another star in which the diversity of size of 

 the components and their proximity render its observation 

 '• . idedly difficult. The student will see both these objects 



tter with a high power than with a lower one. 20 (N.E. 

 ; /) Tauri in the map) is a pretty star, from the con- 

 ■- .isted colours of its components, and is vei-y easy from 



their distance. The coniparison is almost horizontally to 

 the left of the larger star, il c 72 is an equally easy pair. 

 It will be found just to the left of the solstitial colure 

 in the map. 225 P. v. Aurigie, to the N.E. of 26, 

 must be found by fishing, as it is invisible to the 

 naked eye. When in the field of the telescope, liowever, 

 it will be found to be a close and extremely pretty littli' 

 pair. 



W^e may conclude to-night's work by a glance at two or 

 three of the most striking clusters of stars in the constel- 

 lation under review. And first, M. 38 (north of (p Auriga-) 

 forms a beautiful field, the main cluster assuming a cruci- 

 form aspect. The telescope may be moved about- in this 

 neighbourhood, which is a rich one. M. 3G (nearly due E. 

 of (/i) is also very line. M. 37 (N. of the double star 225, 

 previously described) is a glorious f^eld, even with such an 

 instrument as that which we are employing. In regarding 

 a nebula or cluster, no light should be suffered to enter 

 the eye for some little time befoie it is applied to the 

 telescope : and the observer should gaze steadily at such an 

 object until the eye becomes accustomed to it, after which 

 hitherto imperceptible detail will flash up. Another rich 

 field will be found in I^I VII. 33 (marked 33' at the very 

 top of the map). 



Next week we will devote to Geuiini and the constella- 

 tions south of it. 



Erratum. — Page 221, col. 2, line 22, t, Tauri should be 

 ( Tauri. 



THE ELECTRIC TELEGRAPH. 



By W. Lyn-d. 



The Battery. 



BEFORE describuig the instruments used by the 

 Government and the Telegraph Companies for the 

 transmission of news and private telegrams, a brief account 

 of the generators of the electric current is necessary. 



The simplest form of apparatus for producing electricity 

 by chemical action is called the Voltaic cell, named in 

 honour of the inventor, Professor Volta, a celebrated Italian 

 philosopher. The cell is easily made, and the cost of the 

 materials is trifling. Two strips of metal— one zinc and 

 the other copper— a glass cup, nearly filled with water, to 

 which a little dilute sulphuric acid is added, two Viits of 

 copper-wire, and the apparatus is complete. The wii-es 

 must be soldered to the metallic plates. When the metals 

 are immersed in the liquid and the extremities of the wires 

 placed in contact, chemical action begins. 'J'hc currents of 

 electricity are produced at the expense of the zinc and the 

 acid. The zinc is consumed and the water is decomposed. 

 The zinc has a very strong aftinity for oxygen. The 

 clicmical action going on in the cell when the metals are 

 in contact is rendered visible to a certain degree. The 

 liberated hydrogen may be seen collecting in bubbles on 

 the copper plate. The zinc is acted upon by the acid. The 

 oxygen, liberated upon the decomposition of the water, 

 combines with the zinc, forming an oxide of zinc. The 

 copper is not acted upon. 



" The materials of an ordinary voltaic cell," Professor 

 Thompson says, " may be regarded as the fuels of electric 

 currents, just as coke and coal are the fuels of steam- 

 power. Like those fuels, they represent a store of energy.'' 

 In the voltaic cell, the flow of electricity continues so long 

 as the wires are joined and the direction of the current is 

 from the zinc plate through the acidulated water to the 

 copper, and from tlie copper through the wire back to the 

 zinc. A single cell such as I have here described would 



