368 ON SWEETENING SEA-WATER, 



pcndicular direction, the vessel will continually acquire a new acce 

 leration, in the direction of the keei, until her velocity be superior 

 to that of the wind, and that in the ratio of about lour to three. 



It is indeed true, that, as the masts of vessel are placed at pre- 

 sent, it is not possible that the yards can form with the keel an 

 angle less than fotty degrees ; but some navigators assert, that by 

 means of a small change this angle might be reduced to thirty de- 

 grees. In this case, and supposing that the vessel could acquire in 

 the direct line a velocity equal to 3-4 ths that of the wind, the velo- 

 city which it would acquire by receiving the wind on the sails at 

 right angles, might extend to 1'034 that of the wind, which is a 

 little more than unity, and therefore somewhat more than the 

 velocity of the wind. 



If we suppose the same velocity possible in the direct course, and 

 that the sail forms with the keel an angle of 40 degrees, it will be 

 found that the velocity acquired by the vessel, in an oblique course, 

 will be nearly 19-20ths the velocity of the wind. 



This at least will be the case, if in this position of the sails, in 

 regard to the wrhd, they do not hurt or obstruct each other. If 

 all these circumstances therefore be combined, it appears that 

 though it is possible, speaking mathematically, that a vessel can 

 move with the same velocity as the wind, or even with a greater, it 

 will be very difficult to produce this effect in practice. 



[Ilutton. Montuda's Ozanam.] 



SECTION XI. 



On sweetening Sca-Kater. 



As a knowledge of the means whereby fresh or sweet water may 

 be procured from salt water is of the utmost importance to sea. 

 faring men, we shall here offer a few remarks on this subject. 



Sweet water may be obtained from salt water by two methods, 

 by freezing such water, or by distilling it. 



When sea-wntcr is exposed to a degree of cold somewhat below 

 the point at which fresh water freezes, its power of holding muriate 

 of soda and other saline substances in solution, is in part destroyed ; 

 ice is formed on the upper surface, while the fluid portion under- 

 neath becomes a concentrated brine. This ice when melted yields 



