356 



been unable, for want of a sufficient number of specimens of water 

 taken at various depths in the Mediterranean, to draw any certain 

 inference as to what becomes of the vast amount of salt brought into 

 that sea by the constant current which sets in from the Atlantic 

 through the Straits of Gibraltar, and which, on the evaporation of 

 the water, must either remain in the basin of the Mediterranean, or 

 escape by some hitherto unexplained means. In the hope of ob- 

 taining further evidence on this question, he had requested Captain 

 Smyth, R.N., who was engaged in a Survey of that sea, to procure 

 specimens of water from the greatest accessible depths. The speci- 

 mens collected by Captain Smyth were, in consequence of Dr. Mar- 

 cet's death, given to other persons, and applied to other objects. 

 Dr. Wollaston, however, fortunately obtained the three remaining 

 bottles of the collection. 



The contents of one of these, taken up at about fifty miles within 

 the Straits, and from a depth of 670 fathoms, was found to have a 

 density exceeding that of distilled water by more than four times the 

 usual excess ; and accordingly, it left upon evaporation more than 

 four times the usual quantity of saline residuum. The result of the 

 examination of this specimen accords completely with the anticipa- 

 tion, that a counter current of denser water might exist at great 

 depths in the neighbourhood of the Straits, capable of carrying west- 

 ward into the Atlantic as much salt as enters into the Mediterranean 

 with the eastward current near the surface. If the two currents were 

 of equal breadth and depth, the velocity of the lower current need 

 only be one fourth of that of the upper current, in order to prevent 

 any increase of saltness in the Mediterranean. 



An Account of the preliminary Experiments and ultimate Construction 

 of a Refracting Telescope of 7'8 inches aperture, with a fluid con- 

 cave Lens. In a Letter addressed to Davies Gilbert, Esq. P.R.S. 

 By Peter Barlow, Esq. F.R.S. #c. Read December 18, 1828. 

 [Phil. Trans. 1829, p. 33.] 



The author gives an account of the continuation of his experiments 

 on the construction of refracting telescopes with fluid lenses, which 

 the aid furnished him by the Board of Longitude enabled him to 

 pursue. The instrument he particularly describes has a clear aper- 

 ture of 7 '8 inches, which exceeds by about an inch that of the 

 largest refracting telescope in this country. The whole length of 

 the tube, with the eye-piece, is 12 feet, but its effective focus is 18 

 feet. It carries a power of 700 on the closest double stars in South's 

 and Herschel's catalogue, and shows them round and defined. This 

 telescope is mounted on a revolving stand, which works with consi- 

 derable accuracy as an azimuth and altitude instrument. The weight 

 of the stand is about 400 pounds, and that of the telescope 130 pounds, 

 being purposely made heavy in order to obtain steadiness ; yet its 

 motions are so smooth, and the power so arranged, as to be easily 

 manageable by one person ; and the star may be followed by a slight 



