NATURAL HISTORY OP TIDES. 41 



Caraccas from the Canary Islands; ten months to make 

 the circuit of the Gulf of Mexico ; but only forty or 

 fifty days to travel, by the gulf-stream, from Florida 

 to Newfoundland. 



Our hasty sketch of the Ocean must conclude with 

 a brief outline of the natural history of Tides, that 

 semidiurnal pulsation which upheaves and depresses 

 the mass of water with a nicely-regulated motion, 

 varying within very narrow limits, and returning to 

 the first starting-point with the changes in the position 

 of the heavenly bodies. That the tides depend on the 

 attractions of the sun and moon is evident, from the 

 fact that at every return of new and full moon we have 

 high tides, and, at half moon, lower tides. This varia- 

 tion in the tide, according to the " age " of the moon, 

 shows us that she is not the only actor ; for when the 

 tides rise highest she is in a line with the sun, and 

 when they are least raised, her attraction acts at right 

 angles with that exercised by the sun. The tides, in 

 the first case, are the result of the united action or sum 

 of the two forces, and, in the latter, of their opposing 

 action or difference. The first are called spring, the 

 latter neap tides. The moon, owing to her far greater 

 proximity, acts upon the waters of the earth, with a 

 force about two and a half times greater than that 

 of the sun, whose immense distance so far neutralizes 

 his attraction. The exact proportion of the forces is 

 as 58 to 23, and if we take their sum to indicate the 

 greatest, and their difference the least, lifting power, 

 the greatest spring tide compared to the lowest neap 

 tide, ought to be as 81 to 25. But these numbers 



