204 A TREATISE 



absolute distance, though of the utmost consequence 

 in all calculations of the heights of the tide, to which 

 the variations of the state of the atmosphere, occa- 

 sioned by the attraction of the sun and moon, must be 

 analogous. And yet this fact, important as it is to 

 every sea-faring person, especially in river-navigations*, 

 as well as to ship-builders, for predicting the highest 

 spring-tides, seems to be totally unknown to the ge- 

 nerality of these persons; nor is it surprising, as it is 

 not taken notice of in any treatise on navigation that 

 I have met with. But M. De la Lande (Astro- 

 nomy, vol. iii. p. 656.) shews, that if the moon's 

 mean force to raise the waters of the ocean be two and 

 a half, her greatest force when Apogee \ will be three; 

 and her least when Perigee, two : a difference suffi- 

 cient to account for the tides at the quadratures 

 being sometimes nearly as high as those at the sy- 

 zy<nes : a circumstance which was ascertained by part 

 of a committee instituted for examining plans for 

 new powder-works at the Old Fort Ghaut ; where 

 stakes had been driven, en purpose to find the rise of 

 the tide. M. De la Lanpe confirms the theory 

 by many observations, made with great accuracy in 

 some of the ports of France (Supplement, vol. iv.) ; 

 and I can vouch for the fact by numerous measures 

 of the heights of the tide, both at the old and new 

 powder-works. But you may easily satisfy yourself of 

 the fact, by observing the height of a few tides at 

 Champaul Ghaut, when you will find, invariably, 

 that every great parallax of the moon, at the syzygies, 

 is attended with a very high tide and strong bore; 

 and vie: versa. I have not be;n able to observe 



that 



