336 PHILOSOPHICAL TRANSACTIONS. [aNNO I766. 



under each of the white fascia, which admits air into every respective lobe : so 

 that the lobes are not filled at once by inspiration, but successively downwards. 



XXV. Observations on the Variation of the Magnetic Needle, as made on Board 

 the Montagu Man of War, in the Years 1760, I76J, and 1762, by Mr. 

 David Ross, Surgeon. By Mr. W. Mountayne, F. R. S. p. 21 6. 

 The following tables Mr. M. compared with the Variation Chart, published in 

 the year 1756, and found that they agree pretty well in general, making allow- 

 ance for the time elapsed * : it is true, that in some few places in the Atlantic 

 Ocean they differ ; yet this may probably arise, as is often the case, from an 

 error in the Montagu's supposed longitude, where such observations were made. 

 But the greatest difference, a greater than should arise, he thinks, according to 

 common course, appears on the coast of Portugal, Cape Saint Vincent, and 

 about Gibraltar, near and within sight of land, where the observations are as- 

 certained to the spot. Hence if Mr. M.'s observed about the year 1756, and 

 those of Mr. Ross's, were both near the truth, at the respective times when 

 they were taken, he knows not how to account for this considerable increase, 

 unless those late extraordinary convulsions, in the bowels of the earth, on those 

 several coasts, may be found, by further experiments, to have there influenced 

 the directions of the magnetic needle. 



On the Variation of the Magnetic Needle ; with a Sett of Observations made 

 on Board His Majesty's Ship, Montagu, during the Years I760, I761, and 

 1762. By Mr. David Ross, Surgeon, p. 218. 



These observations Mr. R. had an opportunity of making, with great care and 

 accuracy on the deflection of the magnetic needle, by an azimuth compass of 

 Dr. Gowen Knight's construction, during a passage to the West-India islands, 

 on a cruise there, and return to Britain ; and likewise a cruise about the Straits 

 of Gibraltar: usually taking a medium of 6 or 10 azimuths, or more. Mr. R. 

 also remarks with surprize that he always found the variation less at anchor, in 

 the West-India islands, than at sea, though near the same spot. As for instance, 

 June 18, 1760, 3 leagues south of Prince Rupert's Bay, Dominico, he found the 

 variation, by a medium of 5 azimuths and an amplitude, to be 5° 27' E : and in 

 the bay itself, June 20, by 3 azimuths and an amplitude, it was 3° 27'; and on 

 the 22d, by 6 azimuths and amplitude, 3° 12' e. The medium of these is 3° 

 20' E., full 2° 7' less than in the offing, though but 9 miles off. The same phe- 

 nomenon appeared on experiments repeated March 23d 176J, when in the same 

 bay, the medium of 8 azimuths gave 3° Ip' e. Off of Antigua, and at anchor 

 in Saint John's Road, the same thing was observed, though the difference was 

 not so great. 



• By this expression, I do not mean that the variation undergoes a regular and unitbrm alteration, 

 notwithstanding such appearance, but rather suspect the contrary. — Orig. 



