ON XHB TIDES. 2,S9 



V. 



Memoir on the Tides, By Cit. Laplace *. 



1 HE obje6l of this paper is to compare the high tides ob- 

 ferved on the 23d of March laft, with the refuits indicated by 

 the theory of univerfal gravitation. 



At this period the moon was new, and in Iier perigeiim. New moon, m 

 Thefecircuraftances, joined to thofe of an equinodial fyzygy, P^'geum, and 



■^ . . . ^ -' -'"•' equinoclial fy- 



are the mofl favourable to high tides ; and if at fuch a time the zvgy, moft fa- 



adion of the winds fliould combine with thefe regular caufes, ^;ouraWetohigh 



inundations may follow, againft which it is prudent to ufe 



precautions. It is with this view that the board of longitude 



publiflies in the Connoijj'ance des Terns every year, a table of 



the highefl tides that follow every new and full moon. 



To know the real height of the tide produced by the adion Height of tide 



of the fun and moon, and diftinguifli it from that which is ovv- \^^ difference 



, „ . p , . , . . /- «. . between ebb and 



ing to the temporary action or the wmd, it is not iLifiicient to flood. 



obferve the abfolute height at flood, but the correfpondent ebb 



mufl be obferved likewife, and the difference between them 



gives the total height of the tide. We can eafily conceive. Wind ads 



that the adion of the wind muft add to the height of the water, ^1"^''^ °" ''°^^' 



both at ebb and flood, nearly in an equal degree. This con- 



fideration is indifpenfable, for without it all we can deduce 



from obfervation is the fum of the combined effeds, without 



being able to feparate and refer them to their real caufes. 



The tides of the 23d of March were obferved at Breft by Tide of 23d of 

 Cit. Rochon and Mingon. The total height was 23 feet ^"'*'^' 

 four inches, the greafeft ever obferved. That which came 

 rearefl to it, was as far back as the 23d of September 17 14, Sept. 23, 1714. 

 when the moon was full, in her perigeum, and alraoft without 

 declination, as well as the fun. Its total height was 22 feet 

 1 1 inches. 



According to the theory given in the fourth book of La Me- Theory in La 

 canique Celejie, the greateft dilTerence between high and ^ow ^J^^^l^^"\^^^^l 

 water in the preceding fyzygics, is 22 feet 10 inches, which fervatien. 

 differs very little from the obfervations ; But in that book it is 

 remarked, that the local circumftances of every harbour may 

 occafion the relation of the adion of the fun and moon on the 



* National Inftitute of France, IV. 



4 phenomena 



