Proceedings of the British Association. 113 



that the standards which had been made to replace those destroyed 

 by the burning of the House of Commons had been found to alter very 

 slightly but decidedly their dimensions, after having been finished with 

 the greatest care. The other, that cannon were never permitted to be 

 discharged more than 400 times under ordinary circumstances, for 

 after that they were deemed unsafe. — Dr. Robinson stated that a fact 

 which had always appeared very strange to him received a probable 

 explanation from what he had just heard. It was found that the 

 platina standard of a metre which had been constructed under the 

 superintendence of the Academy, and which was a square prism, 

 each of whose four faces, therefore, was entitled to be considered as 

 the standard metre of France, when examined many years afterwards, 

 had no two of its sides of exactly the same length : this was supposed 

 to have arisen from the carelessness of the artist employed in its con- 

 struction, who had accordingly been much censured. He now 

 deemed it highly probable that it had been originally constructed 

 perfect, but had altered its own form. He also mentioned a case in 

 which the glasses of a telescope having been confined by adjusting 

 screws to their places, to which it was necessary for the perfection of 

 the instrument that they should be brought with extraordinary pre- 

 cision, the images of the fixed stars were found to form a cross of 

 light ; and it having occurred to him that the confining screws might, 

 by altering the relative positions of the particles, affect their optical 

 action on the light, it was found upon loosening them, that the irre- 

 gular image disappeared. — Sir D. Brewster detailed many examples 

 of the manner in which pieces of glass, being subjected to strains, by 

 the derangement of their molecular structure, polarized light, show- 

 ing zones of coloured spaces, with dark bands along the neutral parts, 

 where the particles retained their natural arrangement. These deli- 

 cate tints were found by him to vary in course of time ; and a very 

 curious fact was that if cuts were made by a diamond along the dark 

 neutral spaces and the glass then divided, the parts were found to 

 possess a structure precisely similar to the piece of which they were 

 fragments ; but the tints, though exactly similar, were much fainter. 



Friday. 

 ' On the Meteorology of Toronto, and its comparison with that oi 

 Prague, in Bohemia,' by Col. Sabine. — The observations at Toronto 



Q 



