470 THILOSOPHICAL TRANSACTIONS. [aNNO 16Q2. 



and so by reduplication exactly measure out the whole length of the instrument, 

 as in fig. 15; for otherwise a remainder will cause the same inconvenience in 

 this case, as in fig. l6. To which if we add, that a pipe being shortened ac- 

 cording to the poroportions above-mentioned, the sound is raised in the same 

 ratio, it renders the case of the trumpet exactly the same with the monochord. 



As a corollary to this discourse, we may observe that the distances of the 

 trumpet notes, ascending continually, decreased in proportion of 4.4-4.4^10 

 infinitum, for, 



(. second -j Note in the r first •» r "t T f *v, c» ■ 



The .J third j- Table, differs \ second I by ^ 4- V °t^*"^ ^'""S' 

 I fourth J from the I third, &c. J ^ ^ J ^^' 



On the Cause of the Change in the Variation of the Magnetic Needle ; with an 

 Hypothesis of the Structure of the Internal Parts of the Earth. By Mr. 

 Edmund Halley. N° IQS, p. 563. 



Having published, in these Transactions, N° 148, a theory of the variation 

 of the magnetic needle, in which, by comparing many observations, I came at 

 length to this general conclusion, viz. That the globe of the earth might be 

 supposed to be one great magnet, having four magnetical poles or points of at- 

 traction, two of them near each pole of the equator: and that in those parts of 

 the world, which lie near any of those magnetical poles, the needle is chiefly 

 governed thereby ; the nearest pole being always predominant over the more re- 

 mote. And I there endeavoured to state and limit the present position of those 

 poles on the surface of our globe. Yet I found two difficulties not easy to sur- 

 mount : the one was, that no magnet, I had ever seen or heard of, had more 

 than two opposite poles ; whereas the earth had visibly four, and perhaps more. 

 And secondly, it was plain that these poles were not, at least all of them, 

 fixed in the earth, but shifted from place to place, as appeared by the great, 

 changes in the needle's direction within this last century of years ; not only at 

 London, where this great discovery was first made, but almost all over the 

 globe of the earth ; whereas it is not known, or observed, that the poles of a 

 loadstone ever shifted their place in the stone, nor, considering the compact 

 hardness of that substance, can it easily be supposed. 



These difficulties made me quite despair of ever being able to account for 

 this phenomenon, when in an accidental conversation I stumbled on the follow- 

 ing hypothesis. It is sufficiently known and allowed, that the needle's variation 

 changes ; and that this change is gradual and universal, will appear by the 

 following examples. At London, in the year 1580, the variation was observed 



