ON COMMON NOTIONS OP PROBABILITY. 123 



days. When once the notion is obtained that a change 

 of weather will follow that of the moon, the epoch is 

 watched, and the change which is in most instances 

 observed, is admitted as evidence. If any one would 

 carefully note for a considerable time the weather pre- 

 ceding and following prorogations of parliament, he 

 would perhaps astonish the world with a result which 

 no one has yet dreamt of. These cases fall under the 

 third head above described. 



It was frequently supposed, a few years ago, that 

 comets produced hot weather. An examination of the 

 number of comets discovered in years of different 

 average temperature gave it as a result that there were 

 more comets in hot summers than in cold ones. But 

 since hot summers are generally fine, with clear skies, 

 and cold summers cloudy and rainy, it is obvious that 

 the former are more favourable to the discovery of 

 comets than the latter. The fact, then, from which the 

 inference was drawn amounted to this, that the years of 

 heat are those in which we see most comets. With 

 what we know of the matter, there is no more reason 

 to suppose that comets bring heat than that heat brings 

 comets. We must, in all instances of presumed con- 

 nection, look closely at these two distinct things the 

 happening of an event, and our perception of it ; other- 

 wise, we shall always be liable to suppose that an event 

 may produce the first, when it produces only the second. 



There is, however, a class of events which does not 

 appear capable of any of the preceding explanations ; 

 namely, the occurrence of what are called runs of luck 

 at play, and repetitions of similar events in common 

 life, which give rise to such proverbs as this that it 

 never rains but it pours, and misfortunes never come 

 alone. We shall first attempt to destroy the extraor- 

 dinary character of these occurrences, and shall then 

 show that any other order of things would be indeed 

 extraordinary. 



Let there be any event of an unusual character, say 



