rOL. XIX.] PHILOSOPHICAL TRANSACTIONS. 223 



suspended, I described parabolas of several species, whose axes were perpendi- 

 calar, and parameter horizontal, to which the line being applied so as to touch 

 the vertex, it passed very nearly through all the intermediate points of the para- 

 bola, much nearer than the portion of a circle, which passed through the extre- 

 mity of the parameter and latus rectum, would do. 



Hence I conclude, that a ponderous and pliable substance, being suspended 

 on a ring or hollow cylinder, so that its central parts may descend, will form 

 itself into a figure that is more commodious for burning-glasses than the sphe- 

 rical, of which they are now made, being much nearer their most absolute 

 figure, the parabola. 



Now, if a way could be found to give the cloth, or leather, a metalline 

 surface, or a varnish that may bear a good polish ; or if this be found imprac- 

 ticable, perhaps plates of metal may be beat out so thin, as being suspended on 

 a large ring, will by their own gravity receive their true figure ; speculums may 

 be made of any size. And there will be another convenient property in 

 them, that one and the same speculum will be changeable into all degrees of 

 concavity, and so have its focal length increased or diminished, according 

 to the purpose it is designed for. On this ideal devised the following ex-,, 

 periment. 



There was taken a sufficient quantity of potters-clay, of which there was 

 ■ formed a plain circular plate, by help of an iron ring about 13 inches diameter. 

 This was laid on a lesser ring, supported by 4 feet, and it immediately be- 

 came a very regular concave on its upper, and convex on its under surface : 

 but notwithstanding it was set to dry in the shade, yet before it was dry 

 enough its central parts extended so as to become almost plane, not without 

 some defects ; if it had continued in its regularity, I designed to have burned 

 and glazed it in a potter's furnace. I have since had a concave plate of clay 

 which I formed by hand, and glazed, but found the glass to flow more unequally 

 than I expected ; which the potter tells me is caused by the foulness of the 

 earth I used. 



Concerning the Eggs of Snails. By Mr. Anth. van Leuwenhoech. N° 235, 



p. 790. 



Having procured some small, white, round eggs, supposed to be those of 

 snails, mixed in moist earth, Mr. L. separated the eggs from the earth, and 

 kept them apart in a dry place. The consequence was, the eggs soon dried 

 up, and nothing came of them. Convinced of his mistake in separating the 

 eggs from the earth, he procured some more, and inclosed both eggs and earth 

 in a glass tube, open only at one end, which he also close corked, to prevent 



