INTRODUCTIOX. 73 



should have many bottles expressly ready, and 

 throw one or two of them overboard ; for instance, 

 at every fifth degree of latitude. 



The waves are another phenomenon of the mo- 

 tion of the sea ; the theory of this motion is still 

 so imperfect, the object itself so fleeting and diffi- 

 cult to lay hold of, that even general data of the 

 length, breadth, height, and quickness of those 

 masses of water, which assume besides different 

 foims, would be a useful contribution towards the 

 mathematical part of natural philosophy. 



Connected with other nautical operations, and 

 of the utmost consequence to physical geography, 

 is the examination of the depth of the sea. The 

 usual maximum of the soundings of navigators 

 (200 fathoms) is very insufficient, it is true, for 

 such investigations j and hitherto we know of only 

 two instances of measurements of several hundred 

 fathoms, (Phipps Lord Mulgrave, and Peron). 

 The rarity of these experiments seems to arise 

 from the difficulties in performing them ; one of 

 the principal is the uncommonly strong friction 

 which the sounding line experiences on being 

 drawn through the water for such a length, which 

 greatly increases the labour of drawing it up. As 

 it is chiefly the friction of water against water, and 

 not that on the solid body, which is considerable, a 

 thinner, but strong and smooth line, which, to pre- 

 serve it from becoming wet, might be thoroughly 

 imbued with oil, tallow, wax, or a mixture of this 



