1891.] on An Astronomer'' s Work in a Modern Observatory. 415 



an infinitely distant point in space. Thus, in any theory which man 

 with his finite mind can devise, when we talk of originally we simply 

 mean at or during the time considered in our theory. 



Now, Laplace's theory begins at a time, millions on millions of 

 years ago, when the sun had so far disentangled itself from chaos, 

 and its component gaseous particles had by mutual attraction so far 

 coalesced as to form an enormous gaseous ball, far greater in diameter 

 than the orbit of the remotest planet of our present system. The 

 central part of this ball was certainly much more condensed than the 

 rest, and the whole ball revolved. There is nothing improbable in 

 this hypothesis. If gaseous matter came together from different parts 

 of space such coalition would unquestionably occur, and as in the 

 meeting of opposite streams of water or of opposite currents of wind, 

 vortices would be created and revolution about an axis set up such as 

 we are familiar with in the case of whirlpools or cyclones. The 

 resultant would be rotation of the whole globular gaseous mass about 

 an axis. 



Now this gaseous globe begins to cool, and as it cools it necessarily 

 contracts. Then follows a necessary result of contraction, viz. the 

 rotation becomes more rapid. This is a well-known fact in dynamics, 

 about which there is no doubt. Thus, the cooling and the contracting 

 go on, and simultaneously the velocity of rotation becomes greater 

 and greater. At last the time arrives when, for the outside particles, 

 the velocity of rotation becomes such that the centrifugal force is 

 greater than the attractive force, and so the outside particles break 

 off and form a ring. Then, as the process of cooling and contraction 

 proceed still further, another ring is formed, and so on, till we have 

 finally a succession of rings and a condensed central ball. If from 

 any cause the cooling of any of these rings does not go on uniformly, 

 or if some of the gaseous matter of the ring is more easily liquefied 

 than others, then probably a single nucleus of liquid matter will be 

 formed in that ring, and this nucleus will finally by attraction absorb 

 the whole of the matter of which the ring is composed — at first as a 

 gaseous ball with a condensed nucleus, and this will finally solidify 

 into a planet. Or, meanwhile, this yet unformed planet may repeat 

 the history of its parent sun. By contraction, and consequent accele- 

 ration of its rotation, it may throw ofif one or more rings, which in 

 like manner condense into satellites like our moon, or those of Jupiter, 

 Saturn, Uranus, or Neptune. Such, very briefly outlined, is the 

 celebrated nebular hypothesis of Laplace. No one can positively 

 say that the hypothesis is true, still less can any one say that it is 

 untrue. Time does not permit me to enter into the very strong 

 proofs which Laplace urged in favour of its acceptance. 



But I beg you for one moment to cast your imaginations back to 

 a period of time long antecedent to that when our sun had begun to 

 disentangle itself from chaos, and when the fleecy clouds of cosmic 

 stuff had but commenced to rush together. What should we see in 

 such a case were there a true basis for the theory of Laplace? 



