Mr. J. S. Alclis on the Nebular Hypothesis. 309 



of density in the central body, the exterior part would vary in 

 density according to such a law, as that body contracted. 



Those that do not obey this law confirm the nebular hypo- 

 thesis quite as well. They all have satellites, and have them 

 because the detached body was not spherical but elongated, the 

 stalk end of the pear-shaped mass being unusually long. The 

 impossibility of homogeneity necessarily would develope a pear- 

 shaped mass in the cooling down of the central body rather than 

 the ellipsoidal or spheroidal. The density of the earth should 

 be about seven or eight times what the above Table indicates, 

 for it to vary between the inverse square and inverse cube (the 

 law is nearly the inverse cube, but not quite); and to have that 

 increased density with the same amount of angular momentum 

 in the detached body, the latter should be some three or four 

 times as long as broad, and hence in contracting, as it would 

 in length be about double the distance of the earth from the 

 moon, it would naturally separate into two bodies. The moon's 

 original day apparently was nearly half as long again as that of 

 the earth. 



The densities of Jupiter and Saturn are far less than the law 

 would give, and due to the same cause, since they abound with 

 satellites, though the great gap between Mars and Jupiter 

 strongly suggests those nebula? where a central mass is sur- 

 rounded by a ring, on the outskirts of which hang smaller 

 nebulae. 



There is connected with this hypothesis a point in the struc- 

 ture of the earth deserving attention. It has been remarked 

 that there is a tendency in mountain -chains to run north and 

 south, and to present steep slopes to the west, gentle declivities 

 to the east. This may arise from the contraction of the earth. 

 If a portion of unsupported crust sink towards the centre, it 

 will subside on to that which is moving east less rapidly than 

 itself, and in consequence will, so to speak, fall over towards the 

 east, the surface forming a gradual slope to the east, and the 

 fractured western edge a precipitous descent to the west. 



In the moon, too, we see proofs of the contraction continued 

 long after the stage in which we now find the earth. The 

 spheroid of the moon has contracted since it assumed that shape, 

 and, contracting less in the longer diameter, is now more sphe- 

 roidal than it should be according to theory, whilst the thick- 

 ened crust, no longer crushed down on the interior, has left cavi- 

 ties in which the moon's ocean and atmosphere are entombed for 

 ever. 



Manchester Free Grammar School, 

 September 16, 1869. 



