30 Mr Searle, Notes on a Vibration Magnetometer 



Hence the restoring couple due to a small angle 6, when the 

 magnet hangs in a field of strength H, is 



(MH + [x)e or M(H+fH E )6, 

 so that the time, of vibration is given by 



Thus, to compare two fields, we have 



H 1 ±fH A _Tl 

 H 2 +fH, T* 



provided M may be treated as constant. When H E = '1S gauss 

 and (f> is 5°,/H E = 00254 gauss. 



§ 5. The second part of this paper, now to follow, is written 

 with the hope of reviving the use of the ball-ended magnets 

 devised by John Robison, LL.D., F.R.S. Edin., Professor of Natural 

 Philosophy at Edinburgh, from 1774 to 1805 1 . 



Robison gives a brief account of his new form of magnet in an 

 article on " Magnetism " which he contributed to the Supplement 

 to the third edition of the Encyclopaedia Britannica. The Supple- 

 ment was published at Edinburgh in 1801. Robison says 

 (p. 116) : — " We got some magnets made, consisting of two balls 

 connected by a slender rod. By a very particular mode of impreg- 

 nation, we gave them a pretty good magnetism; and the force of 

 each pole seemed to reside almost in the centre of the ball. This 

 was our object in giving them this shape. It reduced the exami- 

 nation both of the attractive and of the directive power to a very 

 easy computation. The result was, that the force of each pole 

 varied in the inverse duplicate ratio of the distance. The error of 

 this hypothesis in no case amounted to T a gth of the whole. In 

 computing for the phenomena of the directive power the irregu- 



1 From the " biographical account " of John Eobison, read before the Eoyal 

 Society of Edinburgh by his successor, Professor Jobn Playfair (Phil. Trans. Roy. 

 Soc. Edin. Vol. vn. pp. 495 — 539), we learn that Eobison was born near Glasgow in 

 1739. He was educated at the Grammar Scbool of Glasgow, and after entering the 

 University of Glasgow as a student of Humanity in 1750, he took his degree in Arts 

 in April 1756. From 1759 to 1761 he served in the Eoyal Navy, and saw active 

 service at the taking of Quebec. On the election of Dr Black to the Professorship 

 of Chemistry at Edinburgh in 1766, Eobison succeeded him as Lecturer on 

 Chemistry at Glasgow, holding this post till 1769. In 1770 he went to Eussia 

 to assist the Empress in " placing her marine on the best footing," and in 1772 was 

 appointed to "the mathematical chair attached to the Imperial Sea Cadet Corps of 

 Nobles, at Cronstadt," being then proficient in the Eussian language. He returned 

 to Scotland in 1774 to succeed Dr Eussell as Professor of Natural Philosophy at 

 Edinburgh, and he held this post till his death in Jan. 1805. 



