1877.] 



Presents. 



555 



" 3. The Bunsen burner removed. The positive rotation gradually 

 diminished till it stopped. No negative rotation. 



" 4. The bulb heated with Bunsen burner. Good negative rotation ; 

 then stopped, and rotated positively till quite cold. 



"5. Bulb covered with a cold glass shade, and a large red-hot ring 

 applied round equatorially. Positive rotation, but not very strong. 



" 6. On removing the shade and ring the positive movement soon comes 

 to rest. 



" 7. Covered with a hot glass shade, negative rotation, with positive, 

 rotation on cooling (the same as 4). 



" 8. Plunged into hot water. Negative rotation. 



" 9. Eemoved from the hot water, and immediately plunged into cold. 

 Positive rotation." 



Eesults nearly identical were obtained with another radiometer de- 

 scribed as " silver radiometer (1327), one side coated with finely divided 

 silver, electro-deposited." 



We must accordingly recognize three distinct conditions under which 

 motion may be obtained in a radiometer, namely : — (1) difference of tem- 

 perature of the two faces, as in a pith radiometer coated on one face with 

 lampblack ; (2) more favourable presentation of one face than the other, 

 as in a radiometer with curved disks ; (3) roughness of surface on one 

 face (if this be really different from 2). These three conditions may be 

 variously combined so as to assist or oppose each other, as the case may 

 be, in producing motion. 



December 20, 1877. 



The Society then adjourned over the Christmas Eecess, to Thursday, 

 January 10, 1878. 



Presents received, December 6, 1877. 



Transactions. 



London : — Clinical Society. Transactions. Vol. X. 8vo. 1877. 



The Society. 



Geological Society. Quarterly Journal. Vol. XXXIII. Part 3, 4. 



8vo. 1877. The Society. 



Institution of Civil Engineers. Minutes of Proceedings. Vol. XLIX., 



L. 8vo. 1877. The Institution. 



Linnean Society. Journal. Botany. Vol. XVI. No. 93. Catalogue 



of the Library. Part 3. 8vo. 1877. The Society. 



Musical Association. Proceedings. Third Session. 1876-77. 8vo. 



The Association. 



