VOL. LII.] THILOSOPHICAL TRANSACTIONS. 607 



shape. Of the 6, when heated properly, and while they were cooling, 3 were 

 electrified plus, and 2 minus at the table surface, and at the collet or opposite 

 surface the 3 were electrified minus and the 2 plus. The 6th, which was the 

 largest, and of an oblong shape, appeared to be electrified plus near one end, 

 and at the other end, in the opposite part, minus. These instances he thinks 

 are further proofs of the law observed in the tourmalin, viz. that the electric 

 fluid flows to or fro, in one invariable line, according to the circumstances at- 

 tending the experiment. See the letter to Dr. Heberden, Phil. Trans., vol. 51. 



From the contrary appearances happening with gems of the same shape, it is 

 now abundantly evident that the direction of the fluid does not depend on the 

 external figure of the gem, but on some particular internal make or constitution 

 of it. And that there is some such natural disposition in all gems affording these 

 appearances, may be collected from another curious specimen of the tourmalin 

 kind; which is green, and formed in long slender crystals with several sides, 

 many of which are found sticking together, and are brought from South 

 America. 



Mr. E. M. da Costa, member of the r. s., furnished Mr. W. with a parcel 

 of these uncommon crystals the 12th of November 17^1, and desired he would 

 try whether they aflbrded the electric effects of the tourmalin. They were exa- 

 mined the same evening, when he was agreeably surprized to find them, not 

 only like tourmalins in regard to electric appearances, but that the direction of 

 the electric fluid moving in them, is always along the grain or shootings of the 

 crystals ; one end being electrified plus, and the other end minus. And that 

 the fluid is more disposed to pass in that direction than in any other, may be 

 further collected from what has been observed on the grain of the loadstone by 

 Dr. Knight; for though the magnetic poles of a natural loadstone may be varied 

 in any direction, yet the same loadstone admits of being made much more mag- 

 netical along the grain than across it. 



LXFJII. Observations on the Tides in the Straits of Gibraltar. By Henry 

 \i^v^m\(^ More, Esq. p. 447- 



That a very strong tide, at the rate of some knots, sets from the strait into 

 and out of Gibraltar bay, from Cabrita and Europa points, is notorious. That 

 this stream out and in by Europa is a mile or better wide, he had frequently 

 observed. At the same time that the tide has been pouring into the bay, round 

 Europa, he had remarked, both from the high shore and in boats, another stream 

 in the ofling, going the contrary way. 



As a further confirmation of his idea, relating to the mid-stream: being on 

 guard at Europa, in the forenoon, there came a Spanish xebeck from the west, 

 with little wind, and in time was becalmed right off a rock he sat on; where he 



