VOL. XXXI.] PHILOSOPHICAL TRANSACTIONS. 521 



By this it appears, that at about 250 or 300 leagues west from the south- 

 head of Calefornia, the east variation diminishes to about ^ of a degree; that 

 for 1300 leagues from thence, the same easterly variation gradually increases to 

 about 12°, where it becomes greatest. And that at the isle of Guam, 500 

 leagues still more westerly, it is again decreased to 5° 40'. 



As far as this single instance can direct us, I am apt to think, that in all that 

 space of sea which lies to the northwards of our track, between Japon and 

 Calefornia, there is an easterly variation, which is still greater and greater as 

 the north latitude increases. But that to the southward of our track, and 

 especially to the southward of the equinoctial, a westerly variation arises, of 

 no great extent or quantity, but which is greatest about 1000 leagues west from 

 the coasts of Peru and Chili, about the same meridians where Capt. Rogers 

 found the east variation smallest. This is agreeable to the theory of the variation 

 I laid down in N° 148 of these Transactions, about 40 years since; and I then 

 expressly mentioned, in my 7th remark on the observations there cited, that 

 there was undoubtedly such a tract of west variation in the southern parts of 

 the South Sea, it being the necessary consequence of the site of the four 

 magnetical poles there supposed, though at that time I wanted experiments to 

 prove it. 



An Addition to the Description of the Art of Diving or Living under Water. 



By Dr. Halley. N° 3d8, p. 177- 



In N° 349 of the Phil. Trans. I sufficiently explained the method I had 

 practised and found effectual to furnish air at any reasonable depth under water, 

 and in any quantity desired, for the subsistence of men that shall have occasion 

 to work on wrecks, or otherwise at the bottom, under a great pressure of water. 

 This I did by means of the diving-bell, which, being from time to time reple- 

 nished with fresh air, I had found sufficient to maintain 5 men for near 2 hours 

 together in 10 fathom water, without the least hurt or inconvenience. But 

 the bell not being to be moved from place to place, unless by moving the vessel 

 from which it was suspended, this was a great impediment to the work to be 

 done below ; and therefore I bethought myself how to enable the diver to go 

 out from the bell to a considerable distance, and to stay a sufficient time with- 

 out it, with full freedom to act as occasion served. And considering that the 

 pressure being greater on the surface of the water in the bell than on any other 

 surface that was higher than it, the air would by a pipe pass from the bell into 

 a cavity of air over that higher surface ; I concluded, that putting on a cap of 

 lead made weighty enough to sink empty, and in form resembling the bell 

 itself, I might by flexible pipes, which a man might carry coiled on his arm, 



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