VOL. XVI.] PHILOSOPHICAL TRANSACTIONS, 385 



the stone ; notwithstanding that the variation has changed at London full 4 

 degrees since this terrella has been in the society's custody ; and perhaps many 

 more since it was marked : and had there been a similar change in the poles of 

 the load-stone, it must needs have been perceived in this, whose diameter is 

 about 4i inches. However, to put this matter past dispute, care was taken 

 to find out exactly, and mark the poles of the society's great load-stone, the 

 sphere of whose activity is above Q feet radius, and whose poles are 13 inches 

 asunder, by which, if this translation of the poles be real, it cannot fail of 

 being made very sensible in future times. As to the supposition that the points 

 in which the iron has received the magnetical virtue may change place, after 

 the same manner as the poles of the earth's magnetism are observed to do ; 

 though it was considered as an ingenious hint, and worth prosecution, yet some 

 of the company, well skilled in magnetics, were of opinion, rather that such 

 a circular needle would librate on its centre, so as to respect the magnetical 

 meridian with the points that had at first received the touch, than that the ring 

 remaining immoveable, the directive virtue in it should be transferred from 

 place to place, either by length of time, or by transporting this compass into 

 those parts where the variation of the needle is considerably different. 



On the great Ejects of a new Burning Speculum, lately made in Germany. 



N° 188, p. 352. •-,.,,..;.■,':'-.• r;.- - • 



The outer circle of this concave burning speculum is near '3 E^ipsic ells in 

 diameter, and was made of a copper plate scarcely twice as thick as the back of 

 an ordinary knife ; and may therefore be easily removed from place to place,' 

 and ordered for use. The polish of it is very good, and represents by distinct 

 reflections, all those appearances which arise from its concave figure ; represent- 

 ing a dwarf like a giant, or the head or other part of a prodigious magnitude. 

 The eye being placed nearer the speculum than its focus, all objects are seen 

 within it, in an erect posture, and as at a great distance ; but the eye being 

 farther off than the focus, all things appear inverted, and without the specu- 

 lum : and because the focus is 2 ells off, it is pleasant to see objects distinctly 

 as it were hanging in the aii^; and if a sword be drawn against the speculum, a 

 spectator not used to such optical delusions, would be apt to be frightened- 

 imagining a pass to be made at his face. 



The force of this speculum in burning is ihcredible. For, 1. A piece of 

 wood, put in the focus, flames in a moment, so as a fresh wind can hardly put 

 it out. 1. Water, applied in an earthen vessel, presently boils; and the vessel 

 being held there some time, the water evaporated all away. 3 . A piece of tin, 



VOL. HI. 3 D 



