318 OF TIDES. 



height, and at this time it is said to be high water. The elevation 

 subsides by decrees, and in about six hours it is low water, the sea 

 having atiaintd its greatest depression; after this it rises again 

 when the moon passes the meridian below the horizon, so that the 

 ebb and flood orour twice a day, but become daily later and later 

 by about 50^ minutes, which is the excess of a lunar day above a 

 solar one, since 28j lunar days are nearly equal to 29| solar 

 ones. 



The second phenomenon is, that the tides are sensibly increased 

 at the time of the new and full moon: this increase and diminution 

 constitute the spring and neap tides ; the augmentation becomes 

 also still more observable when the moon is in its perigee, or nearest 

 the earth. The lowest as well as the highest water is at the time 

 of the spring tides ; the neap tides neither rise so high nor fall so 

 low. 



The third phenomenon of the tides is the augmentation which 

 occurs at the time of the equinoxes: so that the greatest tides are 

 when a new or full moon happens near the equinox, while the moon 

 is in its perigee. The effects of these tides are often still more in- 

 creased by the equinoctial winds, which are sometimes so powerful 

 as to prodiue a greater tide before or after the equinox, than that 

 which happens in the usual course, at the time of the equinox 

 itself. 



These simple facts are amply sufficient to establish the depend, 

 ence of the tides on the moon : tiiey were first correctly explained 

 by Newton as tlie necessary consequences of the laws of gravitation, 

 but the theory has been still further improved by the labours of 

 later mathematicians. The whole of the investigations has been 

 considered as the most difficult of all astronomical problems; some 

 of the circumstances depend on causes which must probably remain 

 for ever unknown to us ; and unless we could every where measure 

 the depth of the sea, it would be impossible to apply a theory, even 

 if absolutely perfect, to the solution of every difficulty that might 

 occur. A very injudicious attempt has been made to refer the 

 phenomena of the tides to causes totally different from these, and 

 depending on the annual melting of the polar ice : the respectability 

 of its author is the only claim which it possesses even to be ineri- 

 tinred : and a serous confutation of so ground less an opinion would 

 be perfectly superfluous. 



