VOL. XIV.] PHILOSOPHICAL TRANSACTIONS. 6/ 



the following paper by Dr. Halley, which renders the reprinting of this tedious 

 description of them quite unnecessary. 



A Theory of the Tides at the Bar of Tonquin. By Mr. Edm. Halley, F.R.S. 



N° 162, p. 681. 



The effect of the moon on the waters in the production of the tides in the 

 port of Tonquin is the more surprising, as it seems different in all its circum- 

 stances from the general rule, whereby the motion of the sea is regulated in 

 all other parts of the world that I have yet heard of. For first, each flux is of 

 about 12 hours duration, and its correspondent reflux as long; so that there is 

 but one high water in 24 hours. Then there are in each month two intermis- 

 sions of the tides, about 14 days asunder, when there is no sensible flood or 

 rising of the waters to be observed, but the sea is in a manner stagnant. Thirdly, 

 that the increase of the water has its 14 days period between the aforesaid inter- 

 missions; and at 7 days end makes the highest tides; from which time the water 

 again gradually abates, and the flood is weaker till it comes to a stagnation, both 

 increase and decrease observing the same rule in being exceedingly slow in their 

 beginning and end, and swift in the middle. Lastly, and which is most strange, 

 the rising moon in the one half of each month makes high water, and the set- 

 ting moon in the other half. 



These particulars considered, together with the tables showing the days of 

 the water's stagnation in each month, gave me a light into the secret of this 

 strange appearance, so as to be able to bring the hitherto unaccountable irregu- 

 larity of these tides to a certain rule. And first it appears that the intermissions 

 of the tides happen nearly on those days that the moon enters the signs of Aries 

 and Libra, or passes the equinoctial, which divides the moon's course nearly into 

 two equal parts, as well as the sun's; and from hence it follows, that the tro- 

 pical moons in 25 and VJ", are those which occasion the greatest flux and reflux. 

 It also appears that the moon in northern signs brings in the flood, whilst she 

 is above the horizon, so as to make high water at her setting, and on the con- 

 trary, that whilst she is in southern signs, it flows all the time the moon is 

 below the horizon, and so makes high water at her rising. But it is to be ob- 

 served, that though the moon pass swiftly from south to north when she is in 

 or near Y"j ^^d from north to south when in or near lii, yet the motion of the 

 sea, which is the cause of this tide, is scarcely discernible for 3 or 4 days, when 

 the moon passes the said equinoctial points; whence it appears, that though the 

 declination of the moon be that whereby these tides are regulated^ yet the 

 increase and decrease of the water is by no means proportionate to that of her 



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