Ch. xi. On the Fruitfulness of Marriages. 493 



flux of strangers, that occasions in towns a greater 

 proportion of marriages than in the country, 

 although there can be little doubt that the pre- 

 ventive check prevails most in towns. The con- 

 verse of this will also be true; and consequently 

 in such a country as America, where half of the 

 population is under sixteen, the proportion of 

 yearly marriages will not accurately express how 

 little the preventive check really operates. 



But on the supposition of nearly the same 

 natural prolifickness in the women of most coun- 

 tries, the smallness of the proportion of births 

 will generally indicate, with tolerable exactness, 

 the degree in which the preventive check prevails, 

 whether arising principally from late, and conse- 

 quently unprolific, marriages, or from a large pro- 

 portion of the population above the age of puberty 

 dying unmarried. 



That the reader may see at once the rate of in- 

 crease, and the period of doubling, which would 

 result from any observed proportion of births to 

 deaths, and of these to the whole population, I 

 subjoin two tables from Sussmilch, calculated by 

 Euler, which I believe are very correct. The first 

 is confined to the supposition of a mortality of 1 

 in 36, and therefore can only be applied to 

 countries where such a mortality is known to take 

 place. The other is general, depending solely 

 upon the proportion which the excess of the births 

 above the burials bears to the whole population, 

 and therefore may be applied universally to all 



