88 SCIENCE PROGRESS 



to a planet or ring of small planets lying between Mercury and 

 the sun, may be also explained in this manner. 



The moon having suffered numerous collisions with smaller 

 satellites has had its surface marked with the round sunken 

 craters which are so distinctive a feature. 



So different a theory from the ordinary volcanic one, 

 however, will not be easily accepted by selenologists. Prof. See 

 considers that the almost perfect circularity of Neptune's orbit 

 shows that it cannot be the outermost planet of our system, 

 the roundness indicating that the nebulous medium was quite 

 dense at that distance, and consequently the limits of the 

 system are much farther out. Other planets lying beyond 

 Neptune have been suspected and may yet be discovered by 

 the telescope. It is remarkable that Prof. Forbes considers 

 that one of these bodies, whose distance he supposes is 

 about a hundred times that of the earth from the sun, and 

 consequently would have a period of a thousand years (by 

 Kepler's third law, squares of periodic times as cubes of 

 distances from 'sun, 1000^= 1,000,000= 100^) moves in a very 

 eccentric orbit, whose plane makes a large angle to that of the 

 ecliptic, the resisting medium at that distance apparently having 

 had little effect on its motion. 



The solar system, in the opinion of Prof. See, was formed 

 from a spiral nebula, the latter arising from the meeting of two 

 or more streams of cosmical dust. The system began to whirl 

 about a central point and thus gave rise to a vortex. Great 

 numbers of spiral nebulae are now known to exist scattered all 

 over the heavens, millions of these objects being visible in the 

 most powerful telescopes. On the other hand, it has been 

 pointed out that there are very few nebulae of the oblate 

 spheroidal form, such as the hypothesis of Laplace assumed, to 

 be met with in the sky. " Such nebulae as we see have, it 

 seems, a greater analogy with the solar corona than with the 

 fiery condensing mists conceived of by Laplace " (Proctor, Old 

 and Nciv Astronomy^ § i445)- 



The rotation period of Mars being about 24 h. 37 m. and 

 that of our own earth 23 h. 56 m., Prof, See considers that 

 the period 23 h. 21 m. for Venus, obtained by the early 

 Italian observers, is probably about its true value, and thus the 

 planet is habitable and probably inhabited by intelligent beings. 



It is well known that periodic comets probably owe their 



