Proceedings of the British Association. 233 



too, to the Egyptian columns, the strength of the column being only 

 that of the smallest part of the section. From the two series of 

 experiments, it appeared that the strength of a short column is nearly 

 in proportion to the area of the section, though the strength of the 

 larger one is somewhat less than in that proportion. 



Prof. Challis inquired whether Mr. Hodgkinson had found the 

 columns to give way chiefly in the direction of the cleavages of the 

 stone ? Mr. Hodgkinson replied that he had ; and that hence the 

 same size and shape of stone cut out of the same block, required very 

 different forces to crush them across the grain from what they did 

 with it. — Prof. Stevelly said, that it was one peculiarity of Mr. 

 Hodgkinson' s researches, that they opened up so many collateral 

 objects of interest and wide fields of inquiry. It was easy to see that 

 the present researches might become important to the geologist, by 

 leading him to the source from which originated the splitting up of 

 extended rocks into beds and strata, and the contortions of them ; 

 for example, to some volcanic matter forced up vertically in such a 

 manner as to exercise a crushing force upon even distant masses.— 

 Prof. Willis shewed, by examples deduced from various styles of 

 architecture, that the ancients must have been practically in posses- 

 sion of similar principles ; and from several examples which he gave, it 

 would appear that columns of a shape suited to these principles were 

 again coming into use. 



1 An Improvement in the Method of taking Positive Talbotypes 

 (Calotypes),' by Sir D. Brewster. — In the method now in use the face 

 of the negative Talbotype is placed directly upon the side of the paper 

 which has been brushed over with a solution of nitrate, or ammonia-ni- 

 trate of silver, and which is to receive the positive picture. In strong 

 sunlight the picture is thus taken very quickly ; but there is a rough- 

 ness in the shades, owing to the formation of black specks, which 

 destroys the softness of the picture, and in portraits gives a disagreeable 

 harshness to the human face. In order to remove this defect, the 

 author first interposed thin plates of glass, with their surfaces some- 

 times ground and sometimes polished ; but, though the divergency or 

 diffusion of the light, passing through the negative picture, produced 

 great softness in the positive, yet the outlines were too indistinct, 

 though the Talbotypes looked very well, when placed at a distance. 



