VOL. II.] PHILOSOPHICAL TRANSACTIONS. ISQ 



own error, when I learned a little while after, by letters from England, that 

 Mr. Burrows,* Anno 1580, had near London observed the declination of the 

 needle to be 11° 11 m. as well as OfFusius and Sennertus : And that. An. l6l2, 

 Mr. Gunter,-}- professor of the mathematics, had in the same place found the 

 declination much diminished, having then found it but six degrees : And lastly, 

 that. Anno l633, Mr. GellibrandJ had found it but four degrees north-east, 

 conformable to my observations. Which did assure me, that those declinations 

 were not constant but had varied. 



And that I might be convinced by myself, I made from time to time experi- 

 ments in divers places, and found still more and more diminution ; so that. 

 Anno l66o, in June, after I had very exactly traced a meridian by many 

 azimuths, before and after noon, with a brass quadrant of six feet diameter, 

 and applied good needles upon it, the one of seven, the other of ten inches long, 

 I found that they declined but one degree or thereabout: And the last year, 1665, 

 I found no more than ten minutes on the same meridian. Upon which having 

 lately applied, since the receipt of the letter, the same two needles, it seems the 

 declination is yet less than the last year. But this I can assure you, that the 

 declination is yet some minutes towards the east, at least at Paris. So that you 

 may upon my word doubt || of the observation of your friend, whom perhaps 

 the meridian or the needle, or the construction and division of his compass, may 

 have deceived to a degree and a half north-west, which he at the present assigns 

 to the declination. But I doubt not but in 12 or 15 years it will be found true 

 what he affirms, as I have prognosticated by my hypothesis, which makes the 

 declination to vary a degree every seven or eight years. ^ 



* \Vm. Burrowes published, l6l4, " The New Attractive, and the Variation of the Compass." 

 -f- Edmund Gunter, an excellent mathematician, was bom 1581. He was author of many inge- 

 nious books and instruments. The land-surveying chain, and the logarithmic lines placed on rules 

 and scales, are called by his name to this day. He became professor of astronomy in Gresham Col- 

 lege, London, where he died in I626, being only 45 years of age. 



I Mr. Henry Gellibrand succeeded Mr. Gunter as professor of astronomy in Gresham College, 

 where he died in 1 636, at 39 years of age. He was strongly attached to the old or Ptolemaic astro- 

 nomy. He wrote, however, some useful books on navigation ; and after the death of Mr. Briggs he 

 had the care of publishing the Trigonometria Britannica of that author. 



II By the favour of the author, it is not conclusive, that because the declination is yet somewhat 

 towards the east at Paris, it must therefore be so at London; since it is known here, that even 

 the variation of "Whitehall differs from that of Limehouse; which two places are but about four Eng- 

 lish miles distant from each other. 



§ According to Henry Bond, the variation " was first found to decrease by Mr. John Mair; se- 

 condly, by Mr. Edmund Gunter j thirdly, by Mr. Henry Gellibrand j fourthly, by himself in 16*40; 

 and lastly, by Mr. Robert Hooke, and others, in l665." This change in tlie variation is continual 

 and universal, but different in all different places and at all times. We shall hereafter have occasion 



