S U M A T R A. 
aided by itsown Weiglit in thedefcent, after having reached the fummit, 
and to that owes its velocity. 
Countries where the furfs prevail , require boats of a particular can- 
ftrudion, and the art of managing them demands the experience of a 
man*s life. All European boats are more or lefs unfit, and feldom fail 
to occafion the facrifice of the people on board them, in the imprudent 
attempts that are fometlmes made to land with them on the open coaft. 
The force of the furf is extremely great. 1 have known it to overfet 
a country vcflTel, in fuch a manner, that the top of the maft has ftuck 
in the fand, and the lower end made its appearance through her bottom- 
Pieces of i.lotk h^tve been taken up from a wreck, twifted and rent by its 
involved motion. 
In fome places the furfs are ufually greater at high, and in others 
at low water, but I believe they are uniformly more violent during the 
fpring tides. 
I fliall proceed to enquire into the efficient caufe of the furfs. The Confiderauons 
* ^ refpetling the 
winds have doubtlefs a ftrong relation to them. If the air was in all ""f* of the 
surf. 
places or equal denfity, and not liable to any motion, I fuppofe the wa- 
ter would alio remain perfectly at reft, and its furf ace even ; ab ft radii ng 
from the gct^^ral courfe of the tides, and the partial irregularities oc- 
cafioned by the influx of rivers. 1 he current or rne air impclls the 
water, and caufes a fwell, which is the regular rifing and fubfiding of 
the waves. This rife and fall is fimilar to the vibrations of a pendulum, 
and fubjeft to like laws. When a wave is at its height, it defcends 
by the force of gravity, and the momentum acquired in defcending, im- 
pells the neighbouring particles, which, in their turn, rife and impell 
others, and thus form a fucceiEon of waves. This is the cafe in the 
open fea ; but when the fwcU approaches the fhorc, and the depth of 
water is not in proportion to the fize of the fwell, the fubfiding wave^ 
inftead of preffing on a body of a water, which might rife in equal quan- 
I • tity, 
