©74 Philosophical transactions. [anno 1694. 



doubt that this is the very best way, could portable telescopes suffice for the 

 work. And could these satellites be observed at sea, a ship at sea might be 

 enabled to find the meridian she was in, by help of the tables M. Cassini has 

 given in this volume, discovering with very great exactness the said eclipses, 

 beyond what we can yet hope to do by the moon, though she seem to afford us 

 the only means practicable for the seaman. However before sailors can make 

 use of the art of finding the longitude, it will be requisite that the coast of the 

 whole ocean be first laid down truly, for which work this method by the satel- 

 lites is most apposite : and it may be hoped that either the true geometric 

 theory of the moon may be discovered, by the time the charts are completed; 

 or else that some invention of shorter telescopes manageable on ship-board, 

 may suffice to show the eclipses of the satellites at sea, at least those of the 

 third satellite, which fall at a good distance from the body of Jupiter, being 

 near three times as far from him as the first. 



The last, but most considerable treatise of this collection, gives the aforesaid 

 tables for computing the motions of Jupiter's satellites, but more especially 

 those, for speedily finding the eclipses of the first or innermost. Wherein 

 M. Cassini has employed his skill to make easy and obvious to all c^^jacities the 

 calculation of them, which is otherwise operose to the skilful, and not to be 

 undertaken by the less knowing, who yet perhaps would be willing to find the 

 longitude of the places they live in. 



On Magnetism, particularly on the Polarity of a Piece of Iron. By Mr. J, C, 



N°214, p. 257. 



It is known that a rod of iron held perpendicular to the horizon, or inclining, 

 the lower end is its north pole, or attracts the south end of a magnetic needle ; 

 and that the same end held upwards becomes a south pole, or attracts the north 

 end of a needle, and repels the south end. The south I call a mutable pole, 

 which may be north or south, according as you hold it. I call a fixed pole, 

 that which does not change, however you hold it: particularly that is a fixed 

 north pole, which, though held upwards, attracts the needle's south end, and 

 repels the north end : and that is a fixed south pole, which, held downwards, 

 attracts the needle's north end and repels the south end. It is known that the 

 magnet makes such fixed poles. But to do it without the magnet, is what I 

 here chiefly consider. 



1. The species of the pole, whether north or south, may be found by passing 

 the iron rod though cork or wood, and then leaving it to swim on water, it 

 will turn to its proper pole. But this way is not nice, being in some cases so 

 slow, that you would think it to be at rest when in motion towards its pole. 



