The author enters into a minute description of the mechanical ap- 

 paratus employed for obtaining the necessary purchases for the various 

 operations which were required, and gives a circumstantial history of 

 his proceedings. Frequent interruptions were experienced from the 

 state of the weather, and the almost incessant agitation of the water, 

 which was often so powerful as to render the diving-bell unmanage- 

 able, and to expose the divers to serious danger. The diving-bell 

 consisted of a one-ton ship's water-tank, with eight inches of iron 

 riveted to the bottom in order to give it more depth, and having at- 

 tached to it 18 pigs of ballast, the weight of which (17 cwt.) was 

 found to be sufficient to sink it, 



As soon as the necessary arrangements had been completed, the au- 

 thor states that he made a minute survey of the bottom., by means of 

 the diving-bell, and ascertained the exact position and shape of all the 

 large rocks which covered the spot where the treasures and stores of the 

 Thetis had been scattered. The shape of the area where the precious 

 metals in particular had been deposited, was an ellipse, of which the 

 two principal axes measured 48 and 31 feet j and large boulders of 

 granite had been subsequently rolled over these treasures, and re- 

 quired being removed before the latter could be recovered. The su- 

 perincumbent pressure of the sea, aided by the huge materials of the 

 wreck of the frigate, which, under the influence of the swell, acting 

 like a paviour's hammer, with enormous momentum, had jammed to- 

 gether the rocks, and produced a strong cohesion between the frag- 

 ments of wood, and the gold, silver and iron. 



The first object was to clear away every portion of the wreck ; and 

 after this had been accomplished, to loosen and remove all the large 

 rocks in succession, beginning with the smallest, and ending with the 

 largest and most unwieldy. Some of these, which they succeeded in 

 rolling from their situations into deeper water, weighed about thirty 

 or forty tons ; and the largest, which required the greatest efforts to 

 move from its place, was computed to weigh sixty-three tons. This 

 last effort served to show, that no part, either of the wreck or the 

 stores, which was of any value, remained behind ; and after fifteen- 

 sixteenths of the property had been recovered, the enterprise, which 

 had so perfectly succeeded, terminated on the 24th of July, and the 

 Algerine returned to Rio de Janeiro on the 1st of August. 



The author subjoins an account of the currents off Cape Frio, and 

 a description of the climate, which seems to have been favourable, for 

 his party suffered but little from sickness, and the expedition was 

 unattended with the loss of a single life. On one occasion the party 

 were visited by a whale, which approached very near the diving-bell, 

 but fortunately changed its course, without doing any mischief. 



A paper was then read, entitled, ee An Account of a Concave Achro- 

 matic Lens, adapted to the Wired Micrometer, which has been named 

 Macro- micro, from its power to increase the primary image of a Te- 

 lescope without increasing the diameter of the wires in the Micro- 

 meter." By George Dollond, Esq., F.R.S. 



