VOL. XXIII.l PHILOSOPHICAL TRANSACTIONS. 655 



experiment on the distempers he said it was proper for ; but there was enough 

 to try whether they had the virtue to correct the taste and smell of senna. 

 Therefore there was infused two drachms of it with as much senna in a chopine 

 of water, and the experiment confirmed the matter of fact. Being desirous to 

 know what species of plant it was, and it being impossible to discover it by the 

 leaves, the Portuguese surgeon had taken so much care to cut them very small. 

 Monsieur Romberg, who had some of it sent him, perceived some seeds swim- 

 ming on the water, in which they were infused, and taking up as many as he 

 could of these seeds, gave them to Monsieur Marchand, who sowed them, 

 from whence grew up a plant, which we need not go to Brazil to seek : it 

 grows in Europe, nor need we go out of France to find it. It is the scrophu- 

 laria aquatica, growing all about Paris. He finished his discourse with advice 

 to physicians, to apply themselves to the knowledge of what grows in their own 

 countries, before they think of going farther. 



Abstract of a Letter from Dr. IVallis to Captain Edmund Halley ; concerning 

 the Captains Map of Magnetic P^ariations ; and other things relating to the 

 Magnet. Dated Oxford, Marj 12,, 1702. N° 2/8, p. Ii06. 



Your magnetical chart fixes the business of magnetic variation, in these seas, 

 for the present time. If similar observations had been made in former ages, 

 and transmitted to us, it would have been of great use. And if such be made 

 m future, from time to time, and recorded ; by which it may appear at what 

 rate the variation varies; it will afford a great insight into the magnetic doctrine, 

 about which we are now so much in the dark. 



The doctrine of the magnet has been mostly improved at Gresham College, 

 or by those related to it, and there conversant, for an age or two last past ; as 

 Blagrave, Gunter, Gellibrand, Gilbert, Norwood, Wright, Brigs, Foster, &c. 

 and of late by yourself. 



I have given some imperfect intimation of it in the Transactions, N° 276. 

 I believe it was about the beginning of the reign of King Charles the First, that 

 Mr. Gellibrand caused the great concave dial to be erected in the Privy Garden 

 at Whitehall, which I think is still remaining ; with great care to fix a true 

 meridian-line ; and with a large magnetic needle, showing its variation from 

 that meridian, from time to time. I think it not amiss if exact observations 

 were now made, whether the meridian be now just the same as it was then. 

 For it is very possible, that the pole of the earth may in time suffer some little 

 variation, though it may not readily be discerned, which may cause an alteration 

 of the meridian-line. And this, if so, will be more discernible nearer the pole, 

 than farther off. And though such provision were made at Whitehall for ob- 



