624 PHILOSOPHICAL TRANSACTIONS. [aNNO 1683. 



small vessels. In these vessels I have seen the sap sink down suddenly at the 

 time of the growing of the straw, when at the same time I saw the sap rise up 

 in the vessels GG, which sap was made mostly of globules; and when the 

 globules came to pass the valves, where the vessels were narrowest, these glo- 

 bules then changed into the form of cones, till they obtained a larger room, 

 and then they retook their former globular shape. 



Fig. 20 are the rising vessels described in their length, being the same with 

 those represented fig. 19, GG. I III are the places where these valves are, and 

 where the vessels are narrowest. 



A Theory of the Pariation of the Magnetical Compass. By Mr. Edm. Halley, 



F.R.S. N° 148, p. 208. 



The variation of the compass, by which I mean the deflection of the mag- 

 netical needle from the true meridian, is of that great concernment in the art 

 of navigation, that the neglect of it renders almost useless one of the noblest 

 inventions mankind ever yet attained to. Now although the great utility that a 

 perfect knowledge of the theory of the magnetical direction would afford to 

 mankind in general, and especially to those concerned in sea affairs, seem a 

 sufficient incitement to all philosophical and mathematical heads, to take under 

 serious consideration the several phaenomena, and to endeavour to reconcile them 

 by some general rule: yet so it is, that almost all the authors, from whom a 

 discourse of this kind ought to have been expected, pass by in silence the diffi- 

 culties they here encounter. And those that mention this variation by affirming 

 it to proceed from causes altogether uncertain, as are the casual lying of iron 

 mines and load-stones in the earth, put a stop to all further contemplation, and 

 give discouragement to those that would otherwise undertake this inquiry. It 

 is true that not long since one Mr. Bond, an old teacher of navigation, put 

 forth a small treatise, wherein he pretends to calculate the variation : but he 

 limits his hypothesis to the city of London, affirming himself, that the same 

 calculus is not sufficient for other places, whereby it appears that his rule is far 

 short of the so much desired general one. 



Now although I cannot pretend perfectly to establish the numbers and rules 

 of a calculus which shall precisely answer to the variations of all parts of the 

 world, yet I suppose it will not be unacceptable to the curious to propose some- 

 thing of a light into this abstruse mystery, which, if no other, may have this 

 good effect, to stir up the philosophical geniuses of the age to apply themselves 

 more attentively to this useful speculation. But before I proceed, it will be 

 necessary to lay down the grounds on which I raise my conclusions, and at once 



