408 THE PRINCIPLES OF SCIENCE. [CHAP. 



hyperbolic comets, which appeared in the years 1729, 

 1771, 1774, 1818, 1840, and I843, 1 an( ^ as on ly about 800 

 comets altogether have been recorded, the proportion of 

 hyperbolic ones is quite as large as should be expected. 



When we attempt to estimate the numbers of objects 

 which may have existed, we must make large allowances 

 for the limited sphere of our observations. Probably not 

 more than 4000 or 5000 comets have been seen in 

 historical times, but making allowance for the absence of 

 observers in the southern hemisphere, and for the small 

 probability that we see any considerable fraction of those 

 which are in the neighbourhood of our system, we must 

 accept Kepler's opinion, that there are more comets in 

 the regions of space than fishes in the depths of the ocean. 

 When like calculations are made concerning the numbers 

 of meteors visible to us, it is astonishing to find that the 

 number of meteors entering the earth's atmosphere in every 

 twenty-four hours is probably not less than 400,000,000, 

 of which 13,000 exist in every portion of space equal to 

 that filled by the earth. 



Serious fallacies may arise from overlooking the inevit- 

 able conditions under which the records of past events are 

 brought to our notice. Thus it is only the durable objects 

 manufactured by former races of men, such as Hint imple- 

 ments, which can have come to our notice as a general 

 rule. The comparative abundance of iron and bronze 

 articles used by an ancient nation must not be supposed 

 to be coincident with their comparative abundance in our 

 museums, because bronze is far the more durable. There 

 is a prevailing fallacy that our ancestors built more 

 strongly than we do, arising from the fact that the more 

 fragile structures have long since crumbled away. We 

 have few or no relics of the habitations of the poorer 

 classes among the Greeks or Romans, or in fact of any 

 past race ; for the temples, tombs, public buildings, and 

 mansions of the wealthier classes alone endure. There is 

 an immense expanse of past events necessarily lost to us 

 for ever, and we must generally look upon records or relics 

 as exceptional in their character. 



The same considerations apply to geological relics. 

 We could not generally expect that animals would be 

 1 Chambers' Astronomy, ist ed. p. 203. 



