322 THE PRINCIPLES OF SCIENCE. 



the great improbability that the number of railway acci- 

 dents per month should be always equal, or nearly so. 

 The public attention is strongly attracted to any unusual 

 conjunction of events, but there is a fallacious tendency to 

 suppose that every such conjunction must be due to a 

 peculiar new cause coming into operation. Unless it can 

 be clearly shown that such unusual conjunctions occur 

 more frequently than they should do according to the 

 theory of probabilities, we should regard them as merely 

 divergent exceptions. 



Eclipses and remarkable conjunctions of the heavenly 

 bodies may also be regarded as results of ordinary laws, 

 which nevertheless appear to break the regular course 

 of nature, and never fail to excite surprise or even fear. 

 Such conjunctions of bodies vary greatly in frequency. 

 One or other of the satellites of Jupiter is eclipsed almost 

 every day, but the simultaneous eclipse of three satellites 

 can only take place, according to the calculations of War- 

 gentin, after the lapse of 1,317,900 years. The relations of 

 the four satellites are so remarkable, that it is actually im- 

 possible, according to the theory of gravity, that they should 

 all suffer eclipse simultaneously. But it may happen occa- 

 sionally that while some of the satellites are really eclipsed 

 by entering Jupiter's shadow, the others are either occulted 

 or rendered invisible by passing over his disk, as seen by 

 us. Thus on four occasions, in 1681, 1802, 1826, and 1843, 

 Jupiter has been witnessed in the singular condition of 

 being apparently deprived of satellites. A close conjunc- 

 tion of two planets always excites surprise and admira- 

 tion, though conjunctions must naturally occur at intervals 

 in the ordinary course of their motions. We cannot 

 wonder, then, that when three or four planets approach 

 each other closely, the event is long remembered. A most 

 exceptional conjunction of Mars, Jupiter, Saturn, and Mer- 

 cury, which took place in the year 2446 B.C., was adopted 



