September 1, 1891.] 



KNOWLEDGE 



175 



interstices of the grid made of a paste of red lead (Pbg O^j, 

 but since then litharge (Pb 0), a lower oxide, has been 

 used for those of the negative plate, and the red lead put 

 only in that plate which was intended to be used after- 

 wards as the positive pole of the battery. Moreover, in 

 recent methods the two plates are "formed" apart, each 

 by itself, and the time the current is sent through the 

 negative plate is much longer than that applied to the 

 positive. 



The old form of secondary battery, that used by Plants 

 and consisting of two opposed plates, one made of spongy 

 metallic lead and the other of lead oxide, may be taken as 

 the typical form of secondary battery, and the essential 

 process which goes on is the transference of oxygen from 

 the oxide of lead plate to the one consisting of metallic 

 lead. Lead sulphate is formed as well as the peroxide of 

 lead, and a considerable portion of the acid in the cell is 

 used up in prodiicing this. The current owes its origin to 

 the presence of two substances, lead and oxygen, which 

 have a chemical affinity for each other, and thus the 

 action does not differ in its nature from that of an ordinary 

 galvanic cell. 



Secondary batteries require periodical renewal, as during 

 their action the coating of lead peroxide becomes deoxi- 

 dized, and both plates approach to the same composition. 

 This re-forming is readily done by sending for some hours 

 a current from some exterior source, such as a dynamo 

 machine, into the cells in a direction the reverse of that 

 of the current which is produced by the battery itself. After 

 this the battery is restored to its original state, and is 

 capable of furnishing a remarkably steady and unvarying 

 current for many hours in succession. 



Accumulators are a valuable source of Electricity where 

 a considerable current is needed for lighting purposes, 

 and are also useful as an alternative source where the 

 dynamo is used, as, in the case of a temporary break-down 

 of the dynamo, the secondary batteries can be switched on 

 to the circuit. For such purposes as the electric pro- 

 pulsion of small vessels at sea, they seem to be the most 

 economical means, and are beginning to be extensively 

 used lor the propulsion of electric launches and tramway 

 cars ; the secondary batteries being used to furnish the 

 current necessary to drive the electric motor. 



Dr. Frankland lit up his house years ago with success 

 by means of secondary batteries, and when he was absent 

 for some months and the light not used, he found on his 

 return the batteries still in good condition, and the light 

 ready to be turned on at once. 



A DOUBLE PLANET. 



By J. E. Gore, F.R.A.S. 



DOUBLE stars are numerous in the heavens, and 

 double nebulie are not uncommon. Even double 

 comets have been recorded, as in the case of 

 Biela's comet, and the faint companions which 

 have been observed in close attendance upon some 

 of the large comets of recent years. The duplicity of one 

 of the satellites of .Jupiter has even been " suspected," 

 but, as far as I know, the suspicion has not been contirmcd. 

 Although many of the planets of the Solar System are 

 attended by satellites, they are usually considered as single 

 bodies. We may however, perhaps, make an exception 

 of this rule in the case of the Earth and Moon, which 

 have been termed " a double planet " for the following 

 reasons : — 



The Moon's volume compared with that of its primary 

 is greater than that of any other satellite of the Solar 



System. The voluins is about J, of the Earth's volume, 

 and its mass about Jj- of that of the Earth. The volumes 

 of the satellites of the other planets bear a much smaller 

 ratio to the volume of the primary. We need not consider 

 the satellites of Mars, which are very minute bodies, and 

 quite insiguificaut in size compared with their primary. 

 The largest of the satelUtes of .Jupiter has a volume only 

 _^_jj of that of the 'giant planet." The largest of 

 Saturn's satellites. Titan, has probably not more than 

 _._i__ of the volume of Saturn. The exact size of the 

 satellites of Uranus is unknown, but judging from their 

 faintness, we m.iy conclude that their volume is small 

 compared with that of their primary. Even the satellite 

 of Neptune, supposed to be the largest satellite of the 

 Solar System, is probably smiU compared with the planet. 

 Taking its diameter at 4000 miles, and that of Neptune at 

 36,000 miles, the volume of the satellite will be only yl-j 

 of Neptune's volume. 



We see, therefore, that the Moon is comparatively a very 

 large satellite. It is, of course, absulutely smaller than the 

 largest satellite of .Jupiter, Saturn's satellite. Titan, or 

 the satellite of Neptune ; but compared with the Earth, 

 which is a small planet (in comparison with .Jupiter, 

 Saturn, Uranus or Neptune), it must be considered as 

 really an enormous satelUte, and in relutice size deserving 

 to rank rather as a small planet accompanying the Earth 

 in its annual journey round the Sim, than as a satellite 

 revolving roimd it. 



Seen from Venus, the Earth and Moon wUl appear 

 more like a " double planet " than a planet with an 

 attendant satellite. From a consideration of the bright- 

 ness of Venus as seen fi-om the Earth, we may form an 

 estimate of the probable brightness of the Earth and 

 Moon as viewed from Venus. To do this it wUl, of course, 

 be necessary to make some assumptions. We should 

 require, in the first place, to know the " albedo," or 

 reflecting power, of the Earth's surface. 



To determine this accurately would not be an easy 

 matter, but if we assume that it has the same " albedo " 

 as the Moon, we may not, perhaps, be very far from the 

 truth. Now Zollner found the " albedo " of Venus to be 

 represented by the fraction 0-50, or about three times the 

 Moon's " albedo " (0-1736). 



Venus, when at her greatest brilliancy, and approaching 

 inferior conjunction, is distant from the Earth about 39 

 millions of miles, and has then about one-fourth of the area 

 of her disc illuminated by sunlight. The Earth when in 

 " opposition," and therefore at its brightest as seen from 

 Venus, is distant from the planet about 26 millions of 

 miles. Hence we have the relative distances in the ratio 

 of 39 to 26 or as 3 to 2. 



If, to simplify the calculation, we assume the diameter 



of the Earth and Venus as equal, the apparent areas of 



their discs will be as 3- to 2- or as 9 to 4. That is, the 



area of the Earth's disc when in "opposition," as seen 



fi-om Venus, will be about 2| times the area of Venus's 



disc when at her brightest as seen from the Earth. Now 



as the Earth shows a full face to Venus when at its 



brightest, and Venus only one-fourth of a fully illuminated 



disc when at its brightest to us, we should have the Earth 



brighter than Venus in the proportion of 36 to 4, or as 9 



to 1, (/ the distances of both planets from the Sun and 



their " albedos " were the same. But as their distances 



from the Sun are in the ratio of 93 to 67, ^'enus will be 



more brilliantly illuminated in the i-atio of 93- to 67-, or 



I about as 19 to 10, and as its " albedo, " as assumed 



i above, is three times greater, we have the brightness of 



j Venus's surface greater than that of the Earth's surface 



I in the ratio of 57 to 10. Hence, finally, we have the 



