168 Proceedings of the British Association. 



client for securing adhesion. He first precipitated gold from the 

 cyanide of gold, and he mixed with it cyanide of copper, and 

 kept gradually increasing the quantity of the latter sort ; so that 

 an alloy was precipitated, which was continually increasing the 

 copper with respect to the gold, till he had a speculum whose 

 surface was gold, and which then became an alloy, the quality 

 decreasing, till at the bottom it became pure copper. This was 

 important : because, without such experiments, one would not 

 have known that such results would have followed ; for some 

 philosophers supposed, that if we attempt to precipitate the salts 

 of two metals, only one was precipitated ; but Prof. S. informed 

 him that they precipitated in union. He thus obtained a specu- 

 lum with a face of gold and a back of copper. But supposing 

 the largest, cheapest, and best speculum were obtained, the frame- 

 work of the telescope would be so gigantic, that few observers 

 would be able to use the instrument. With a focal length of 

 sixty to eighty feet, it would be quite unmanageable for any pri- 

 vate individual. The idea occurred to him (Mr. T.) to have a 

 tube fixed in an invariable position, and to have a perfectly true 

 plane mirror, of a size somewhat larger than the concave specu- 

 lum, placed in front of the tube, with an aperture in the centre. 

 This plane reflector should be movable about its centre in any 

 direction : so that luminous bodies, falling first upon the plane 

 reflector, were then reflected against the concave reflector and 

 passed through the aperture. The only motion requisite for the 

 plane mirror would be one about its centre. The mechanical 

 difficulties in the way of this plan would be far less than in the 

 common method. Prof. S.'s idea on this point was somewhat 

 different. He (Mr. T.) did not think important in what direc- 

 tion the tube of the telescope was placed. Prof. S. ? s idea was, 

 that it should be pointed directly to the pole of the heavens, and 

 kept as steady as possible, and that the plane mirror should have 

 a simple motion of revolution, indeed two motions, but about a 

 rectangular centre. 



Experimental inquiries on the strength of stones and other 

 materials ; by Mr. Eaton Hodgkinson. Numerous experiments 

 on wood, sandstones, marbles, glass, slate, ivory, bone, &e. had 

 been made by Mr. H. in order to ascertain the tensile, crushing, 

 and transverse strength of each j also, as far as possible, the situ- 

 ation of the neutral line. The following is a summary of the 



