Mechanical Science. 163 



to the plain of the Till, but also to carry away a bridge then in 

 progress of building-, some of the arch-stones of which, weighing 

 from one half to three-fourths of a ton each, were propelled two 

 miles down the rivulet. 



On the same occasion, the current tore away from the abutment 

 of a mill-dam, a large block of greenstone porphyry, weighing 

 nearly two tons, and transported it to the distance of a quarter of 

 a mile. Instances are related to occur repeatedly, in which from 

 one to three thousand tons are in like manner removed to great 

 distances in one day ; and the author asserts, that whenever 400 

 or 500 cart-loads of this gravel are taken away for the repair of 

 roads, that one moderate flood replaces the amount of loss with the 

 same quantity of rounded debris. 



Parallel cases of the power of water are stated to occur in the 

 Tweed, near Coldstream. — Phil Mag. N. S. vi. 65. 



6. Geological Hammer. — Requiring a geological hammer, uniting 

 the properties of power and compactness, Mr. Clarke selected one 

 of Knight's trimming hammers, marked No. 5, and removing the 

 handle from it, supplied another, formed in the following manner ; 

 — Three narrow slips of soling leather, of convenient length, were 

 glued firmly together, so that the grain of the exterior pieces was 

 in a different direction to that of the interior piece. The whole 

 was rounded, and reduced from the extremity upwards, until it 

 fitted the aperture in the hammer head, and there firmly secured 

 by two small wooden wedges, driven in between the slips of leather. 

 The handle was then strongly bound round with twine, from one 

 end to the other. It was founal to possess an elasticity almost 

 perfect, and to preserve its shape. — Mag. Nat. Hist ii. 247. 



7. Cement for hard Stones, Porcelain, and Glass. — This cement 

 is a natural product, which, without being abundant, is in sufficient 

 quantity for all ordinary uses. The large snails which are found in 

 gardens and woods,[and are sometimes used for food, have a vesicle 

 at the extremities of their bodies filled with a whitish substance, 

 having a greasy and gelatinous appearance. If it be applied be- 

 tween two surfaces, whatever be their hardness or compactness, and 

 the surfaces be brought together throughout, so strong an adhesion 

 is ultimately occasioned, that if violent blows or thrusts be given 

 to the substances, they frequently break elsewhere than at the 

 junction. A flint about the size of the fist having been broken into 

 two pieces, and rejoined by this means, being thrown with violence 

 on the pavement, broke into fragments by fresh fractures crossing 

 the former junction, but not going along it. All that is necessary 

 to give this cement its full power is to allow it time to dry. — Bull. 

 Univ. E. xii. 107. 



8. Qn the Structure of Metals, by M. Savart— M. Savart has 



M 2 



