i 



t» On prefsrvtng Vrejh tValer fiueei. 



rifm feemed to have likewife very flight fecondary planes J 

 ait, when that happens, they arc always very narrow, tfpe- 

 daily when compared with the fecond'ary plane of the third 

 edge. This modification, in its various forms, is the moft 

 common one of this fpecies. 



Sometimes one of the folid angles of the triedral prifm is 

 replaced, on one fide only, by a pPane that is much inclined 

 to the edge of the prifm on which it is fituated ; but the cryf- 

 lals are always too fmall to admit of being meafured with 

 precilion. (Fig. 15.) This plane, affuming" a more confi- 

 derable extent, replaces the fame edge of the prifm by an- 

 other plane, much broader at one extremity than the other, 

 as is lliown by the lines of large dots in the fame figure. 

 Sometimes it has a very confiderable extent, as is reprefcnted 

 in fig. 1-6. It then reduces one of the bafes of the triedral 

 prilm to the form of a very narrow trapezium, while that of 

 the oppofite bafe remains very broad. By a ftill more confi- 

 derable incrcafe, the plane of this bnfe totally difappears, and 

 thecryftal is terminated atthat extremity by a r'ldge. (Fig. 17.) 

 In this variety, the cryftal is often feen placed upon one of 

 its fc-alene triangular fides, and then prefents the other at its 

 upper extremity; an appearance which is apt to puzzle the 

 obferver, particularly when he perceives, among the trian- 

 gular fides which moli generally terminate the cryftals, no- 

 thing but equilateral triangles. This modification', in all its 

 torms, is much lefs common than the preceding onej it is, 

 however, occafionally met with. 



[To be coiitiivjed.] 



II. On frefervius: Fre/h Wafer Jweet during long-Voyages^ 

 By Samuel Bkntham, ^j'q* 



X HE Society for the Encouragement of Arts, &c. having 

 thought proper to ofter a premium in order to afcertain, for 

 the ufe of the public, the beft mode of preferving frefh water 

 fweet at fca, 1 requeft you to lay before the Society an ac- 

 count of the method which I have employed for this purpofe 

 on board two fhips, and which has been attended with all 

 the fuccefs that can be reafonably expected. 



The mode in which I conceived frefh water might be pre- 

 ferved fweet, was merely by keeping it in veflels of which 

 the interior lining at leaft fhould be of fuch a fubftance as 



•• From the Tranfaff'tons of the Sociclj of Arts, isfc. Addphi, London, 

 for 1 801. —The Society awarded their gold medal to Mr. Bentham for 

 this communication. 



ihould 



