54 THE POPULAR SCIENCE MONTHLY. 



ing and significant exception to the general tractableness of the plan- 

 ets. Mercury, the nearest to the sun, and the one, therefore, which 

 ought to be the best behaved of all, is rebellious to a certain extent ; 

 the perihelion of its orbit moves around the sun more rapidly than 

 can be explained by the action of the other known planets. The evi- 

 dence to this effect has been continually accumulating ever since Le- 

 verrier first announced the fact, some thirty years ago ; and the recent 

 investigation by Professor Newcomb, of the whole series of observed 

 transits, puts the thing beyond question. Leverrier's own belief (in 

 which he died) was, that the effect is due to an unknown planet or 

 planets between Mercury and the sun ; but, as things now stand, we 

 think that any candid investigator must admit that the probability of 

 the existence of any such body or bodies of considerable dimensions 

 is vanishingly small. We do not forget the numerous instances of 

 round spots seen on the solar disk, nor the eclipse-stars of Watson, 

 Swift, Trouvelot, and others ; but the demonstrated possibility of error 

 or mistake in all these cases, and the tremendous array of negative 

 evidence from the most trustworthy observers, with the best equip- 

 ment and opportunity, make it little short of certain that there is no 

 Vulcan in the planetary system. 



A ring of meteoric matter between the planet and the sun might 

 account for the motion of the perihelion ; but, as Newcomb has sug- 

 gested, such a ring would also disturb the nodes of Mercury's orbit. 

 It has been surmised that the cause may be something in the distribu- 

 tion of matter within the solar globe, or some variation in gravitation 

 from the exact law of the inverse square, or some supplementary elec- 

 tric or magnetic action of the sun, or some special effect of the solar 

 radiation, sensible on account of the planet's proximity, or something 

 peculiar to the region in which the planet moves ; but thus far no sat- 

 isfactory explanation has been established. 



Mercury as yet defies all our attempts to ascertain the length of its 

 day and the character and condition of its surface. Apparently the 

 instruments and methods now at command are insufficient to cope with 

 the difficulties of the problem ; and it is not easy to say how it can be 

 successfully attacked. 



With Venus, the earth's twin-sister, the state of things is a little 

 better : we do already know, with some degree of approximation, her 

 period of rotation ; and the observations of the last few months bid 

 fair, if followed up, to determine the position of her poles, and pos- 

 sibly to give us some knowledge of her mountains, continents, and 

 seas. 



It would be rash to say of Mars that we have reached the limit of 

 possible knowledge as regards a planet's surface ; but the main facts 

 are now determined, and we have a rather surprising amount of sup- 

 posed knowledge regarding his geography. By " supposed " I mean 

 merely to insinuate a modest doubt whether some of the map-makers 



