574 EEPORT — 189i. 



a bar at various temperatures the variation of the modulus of elasticity with 



temperature can be obtained. We observe N at various known temperatures of 



W 

 the bar, whence V is computed, and the moduhis is M = ' As I, t, and d have 



. .'/ _ 



different values at different temperatures, the coefficient of expansion of each 

 bar experimented on was determined, also tJie density of each bar at 40° C, and 

 the dimensions and density computed for each temperature of the vibrating bar. 



Though the moduli thus obtained are not so accurate as those given by other 

 more precise methods, yet I think that the variation of the modulus with tempera- 

 ture is thus obtained. 



Tables and curves of results of the experiments on bars of glass, cast steel, 

 Bessemer steel, brass, bell-metal, two specimens of aluminium, silver, and zinc were 

 exhibited before the Section. 



The moduli of these substances are lowered by increase of temperature as 

 follows. Heating from 0° to 100° lowers the modulus of 



Silver heated from 0° to 60° has its modulus lowered 2'47 per cent, of its 

 modulus at 0°. Zinc heated from 0° to 62° has its modulus lowered 6*04 per cent, 

 of its modulus at 0°. 



9, 0)1 an A'pjyaratus for Measuring small Strains, 

 Jhj Professor J. A. Ewing, F.R.S. 



10. On Mirrors of Maynetism. 

 Bij Professor Silvanus P. Thompson, D.Sc, F.R.S. 



Tlic author has found by experiment that a sheet of iron, if sufficiently large 

 and thick, acts magnetically as a mirror, giving virtual images of a magnet pole 

 placed in front. 



The image of a pole placed at any distance in front (provided the mirror is 

 largo as compared with the distance) is situated at an equal distance behind tlie 

 mirror, exactly as in optical reflexion by a plane mirror. 



The image of a north pole is a south jiole, just as the image of a right hand is 

 a left hand. 



If the magnet pole \'^ moved away from llie sheet of iron, the image moves 

 away at an equal speed in the reverse direction. 



if till' .-~heet of iron be moved up toward the magnet pole, the image moves up 

 at double the velocity, as optical images do. 



If a magnet pole is placed between two parallel large shests of ii-on, the effect 

 is tiie same as if there were a double series of images in an indeiiuitely extended 

 row, the images being, however, of alternate polarity. 



The acti")! of a spherical surface of iron as a spherical mirror presents some 

 curious jioint.o. Only virtual images are produced whether the mirror be convex 

 or cdncave. There is no spherical aberration, and the images, though of reversed 

 ixilarity and perverted, are not inverted. These last points are deduced from the 

 cxpeviinonts, and have not yet been independently verified. 



The metliod of experiment has been to produce magnetic poles or fields by 

 means of coils supplied with electric currents. These poles or fields werj investi- 

 jrated by means of an exploring coil or coils connected with a galvai Ojieter, the 

 throw of which was observed when the primary current was turned on or off. 

 The throw obtained when the min-or was absent was compared (r)with that 



