S Xl M A r K A. 
except at particular periods, are very unfrequent in the Indian Seas ; 
where the navigation is well known to be remarkably fafe ; whilft the 
furfs are almoil continoal ; and that gales are not found to produce this 
effeiwl in other cxtenfive oceans. The weft coaft of Ireland borders a 
fea, nearly as extenfive, and much more wild, than the coall of Sumatra, 
and yet there; though when it biows hard, the l^.vcll on the Ihorc 
is high and dangerous ; is there nothing that referables the furfs of 
India. 
Thefe, fo general in the tropical latitudes, are, upon the moft pro- ^/^^g^^^"^* 
bablc hypothecs I have been able to form, after long obfervation, and 
much thought and enquiry, the confequcnce of the trade or perpetual 
winds which t>ervail, at a diftance from iliore, between the parallels of 
ten and thirty degrees north and touth, wftole unirorm and invariable 
action caufcs a long and conftant fwell, that exifts even in the calmeft 
weather, about the line, towards which its dirediion tends from either 
fide. This fwell or lib ration of the fea, is fo prodigioully long, and the 
fenfible effect of its height of courfe fo much diminifhed, that it is not 
often attended to ; the gradual flope engrolling almoil the whole hoiizon,. 
to an eye not very much elevated above its furface; but perfons who 
have failed in thofe parts may recolle^l that even when the fea is appa- 
rently the moft ft ill and level, a boat or other obje<ft at a diftance from 
the ihip, will be hid from the fight of one looking towards it from the 
lower dcek, for the fpacc of minutes together. This fwell, when a 
fquali happens, or the winti ncniLuo u^/, ^ui, rk** kave other 
fubfidiary waves on the extent of its^ furface, breaking often in a 
direction contrary to it, and which will again fublide as a calm returns^, 
without having produced on it any perceptible effed:. Sumatra, though 
not directly expofcd to the fouth eaft trade,, is nat fo diftant but that its 
influence may be prefumed to extend ta it, and accordingly at Pcoh 
Pifang near the fouth ern extremity the illand, a conftant foutherly 
fea is obferved, even after a hard northweft wind. This inceflant and 
powerful fwell rolling in from an ocean, open even to the poie, fecms- 
an agent adequate to the prodigious effefts produced on the coaft;. 
whUft 
