68 PHILOSOPHICAL TRANSACTIONS. [anNO 1684. 



declination, that changing swiftly, where the increase of the water is observed 

 to be most slow. It seems therefore, and I propose it as a probable conjecture, 

 that the increase of the waters should be always proportionate to the versed sines 

 of the doubled distances of the moon from the equinoctial points; upon which 

 hypothesis, fig. 8, pi. 2, will give an elegant synopsis of the whole matter. 

 Let AB be the bottom of the bar of Tonquin, CD a perpendicular thereto, 

 whereon to measure the several depths of the water; Cy Czdz the mean depth, 

 which is that whereat the water is stagnant on the moon's being on the equi- 

 noctial points, which is commonly about 15 feet; Cgs occid, the high water 

 mark when the moon is in 25 or VJ being about 24 feet; CVJ occid the height 

 of the low water mark when the moon is in 25 or VJ* being about 6 feet; so 

 that the greatest rise of the water on the tropical moons will be about 18 feet; 

 then dividing T 25 and ^ VJ" into two equal parts in E, F, on those two points, 

 as centres, describe the two circles, each of whose radii are 4-l feet, which 

 being kept between the compasses, naturally divide the said circles in the points 

 b n25Sl» &c. through which points if we draw lines parallel to the base AB, 

 they shall cut the perpendicular CD, in the heights of the high and low water 

 marks, which will be at the entrance of the moon into the said signs. So the 

 greatest depth of the high water when the moon enters ti , W.^ Q.i H> is but 

 1 74- feet, and the least at low water 12-5- feet ; but when she enters 11 , SIj ^ , ^j 

 the high water depth is 21 f feet, the low water but 8^ feet, as appears by the 

 figure. And this hypothesis not only agrees with all that Mr. Davenport has 

 observed himself or collected from the natives, but has been found to hold true 

 since, in the year l682, by the ingenious Captain Knox, in his voyage to this 

 port, so that there is no room to doubt of the truth thereof. By this method 

 then may the time and height of the tides be with sufficient certainty com- 

 puted; but to philosophize thereon, and to attempt to assign a reason, why the 

 moon should in so particular a manner influence the waters in this one place, is 

 a task too hard for my undertaking, especially when I consider how little we 

 have been able to establish a genuine and satisfactory theory of the tides, found 

 upon our own coasts, of which we have had so long experience. It would be 

 however a very acceptable thing if some curious navigators would inform us, 

 what tides or currents are found at Macao, Quemoy, and other places on the 

 coast of China, and on Formosa ; it being most probable that this flood comes 

 out of the north east, along the coast of China, for the northerly monsoon is 

 found to occasion the highest spring tides. There is yet another thing well 

 worth inquiry, viz. seeing that this motion of the sea is more or less, as the 

 moon is farther from or nearer to the equinoctial, it is not unlikely that some 

 years may have much higher spring tides than others, according to the various 



