On the Strength of Beams. 191 



that the road is repaired about New)Dort with an amygdaloid, but 

 I had never an opportunity of visiting the spot from whence it 

 was brought. My friend Dr. Prichard, however, informs me, 

 by letter, that it is quarried from a hill at Micklewood to the 

 east of Newport, a few miles beyond the northern boundary of 

 the limestone ellipsis. The hill itself is of very inconsiderable 

 extent, and perfectly unconnected with the surrounding high 

 country, which to the north of it is of oolite; and in walking 

 south of it we first come on the red sandstone, then on the sand- 

 stone conglomerate, both of which we find further south at 

 Cromhall, dipping under the limestone. — From this account, 

 there can be no doubt, I think, that the amygdaloid rests upon 

 the red sandstone. The specimens that I have seen of the amyg- 

 daloid consist of a basis of a sort of iron-clay containing al- 

 mond-shaped portions of calc spar and green earth. 



With regard to the point at issue between Mr. Farey and me, 

 it would be a most unprofitable waste of time to dissect his ses- 

 quipedalian sentences, and to answer each part separately: I 

 shall content myself with stating, that after reading Mr. Farcy's 

 letter, I referred to the original account of Mr. Strachey in the 

 Philosophical Transactions, on the coal-pits near the Mendip 

 Hills, and find the unconformable position of the red ground is 

 most distinctly mentioned both in his section and descrij)tion; so 

 that the merit of having first observed this appearance belongs 

 neither to Mr. Farey nor to myself, but exclusively to Mr. Strachey. 

 My ignorance of this circumstance arose from not having had an 

 opj)artunity, while writing my paper, of consulting the Philoso- 

 phical Transactions themselves, but was obliged to content my- 

 self with referring to Dr. Thomson's Abridgement, where the 

 particulars relative to the occurrence of tiie coal are accurately 

 given, l)ut this point of the unconformable position of the red 

 ground is altogether omitted. 



XXXIV. On the Strength of Beams. By A Correspondent. 



To Mr. Tilloch. ■ 



Sir, — JjUFFON found by his experiments, that the strength of 

 beams decreased in a much quicker jjroportion than the inverse 

 ratio of their lengths. The cause has not been satisfactorily ex- 

 plained ; but may it not be accounted for by the tendency of the 

 weight to compress the l)eam, in the direction of its depth, at 

 the place of fracture? To ascertain the effect of this kind of 

 pressure, I fixed one end of a bar of deal | of an inch square, 

 and suspencjed a weight of 17 lbs. at 12 inches distance from the 



point 



