130 



of these have probably been simply mechanical effects of the inter- 

 nal and external pressures not being at first in equilibrio. 



3. Mr Smith of Jordanhill made a verbal communication 



regarding a change in the Relative Levels of the land 

 and sea on the west coast of Scotland ; recent shells 

 having been found even at the height of 70 feet. 



4. Professor Forbes communicated the following memoran- 



dum respecting the Polarization of Heat. 



" I have recently ascertained the following facts respecting heat. 

 " 1st, That heat polarized in any plane, and then reflected from 

 the surface of a refracting medium, changes its plane of polariza- 

 tion in a manner similar to what obtains in the case of light. Thus, 

 with a thick plate of mica, which polarized homogeneous red light 

 most completely at an incidence of about 59°, the plane of polari- 

 zation of reflected polarized heat remained on the same side of the 

 plane of reflection when the incidence was great, and was on the 

 contrary side when the incidence was small. The limiting angle of 

 incidence was about 37°, which therefore should be the polarizing 

 angle of dark heat for mica. This mode of observing the polarizing 

 angle offers some advantages above more direct methods. 



" 2dly, Metals polarize heat extremely feebly by reflection. I 

 have carried my experiments up to 85° from silver, yet even there 

 but a small share is polarized. The effect is, however, distinctly 

 recognizable through a considerable range of incidences. The ef- 

 fects are such as would indicate the maximum polarizing angle to 

 be even much higher ; perhaps it never attains a ra.iximum. This 

 fact corresponds to that in the case of light, except that the maxi- 

 mum polarizing angle is 73° (Brewster). Did metals act on light 

 like other bodies, we should conclude, from the polarizing angle 

 being greater, that heat is more refrangible than light. An import- 

 ant remark of Sii" D. Brewster's, however, shews the statement I 

 have made to be in conformity with the views of the nature of heat 

 which I have published. He finds the maximum polarizing angle 

 to be greatest for the least refrangible rays. 



" 3dly, Heat polarized in a plane inclined 45° to the plane of sub- 

 sequent reflection at silver, has its nature changed as in the case of 

 light, and presents the conditions of elliptic polarization, though the 



