Tides 
From time immemorial men have observed the tides and 
wondered about them. Some of the ancient scholars recog- 
nized the relation between the moon and the time of tide, but 
knowing nothing about gravitation were unable to account 
for the connection. 
Crude theories about the tides were prevalent until rela- 
tively recent times. Gilbert of Colchester (1544-1623) 
thought the tides were the result of the earth's breathing! 
And he was not an unscientific man for his time. He was the 
author of De Magnete, a historic treatise on the magnetism 
of the earth which for the first time laid down the principles 
that later were to be used in the theory of the marine com- 
pass. John Kepler (1571-1630), the father of modern astron- 
omy, held the same view. Kepler was an astute man of 
science; he computed the first accurate orbits of the planets 
of the solar system, but he, too, thoughi the periodic rise and 
fall of the ocean level was due to the earth’s being alive and 
breathing. It was not until Newton (1642-1727) propounded 
the principle of universal gravitation, and the theory of tides 
was ceveloped from work of many scientists including 
Count Lagrange (1736-1813) and the Marquis de Laplace 
(1749-1827), that the nature of the tides was fully appre- 
ciated. 
WHAT CAUSES THE TIDES? 
The waters of the oceans are held to the surface of the 
earth by the gravitational force that the earth exerts upon 
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