Mechanical Science. 383 



In looking down the last five columns, it will be readily observed 

 how the time diminishes as the separating force of the pendulums 

 (expressed in the first five columns) increase, the original interval 

 between the surfaces being the same ; this is a natural conse- 

 quence of known mechanical laws. But in looking along them 

 horizontally, it will be observed how rapidly the times of the 

 oscillations diminish as the original interval between the sur- 

 faces, as measured by the intervening wires, is increased, although 

 the separating power is but very little altered. 



Without following M. Girard further, we may at once quote his 

 conclusion, and refer our readers for further details to the me- 

 moir itself, in vol. xxix. of the Annates de Chimie. H Those sur- 

 faces which, being entirely immersed in a fluid, are susceptible of 

 being wetted by it, when brought sufficiently near to each other 

 to allow of the mutual penetration of their moistening layers, 

 exert an attraction on each other by means of the interposed 

 liquid, at distances which are sensible and capable of accurate 

 measurement, and which increases as the distance is diminished.'* 



It appears that attention to temperature is of great importance 

 in these kind of experiments. M. Girard found that other cir- 

 cumstances being equal, a variation of temperature from 5° to 20° 

 centigrade (40° to 68° F.) caused a variation in the time from 783" 

 to 520" only. 



S. On an unnoticed Mechanical Principle — Explanation of the 

 cutting of Steel by Iron. — The observation made by Mr. Barnes, 

 in America, of the action of soft iron upon steel, has called forth 

 the experimental remarks of many persons, and numerous expli- 

 cations of it have been given, none of which, however, have been 

 satisfactory to all persons. 



The following is an abstract of parts of a paper by M. Allou : 

 it has been inserted in the Bibliothcque Universelle, xxix. p. 192. 

 It is remarked, that if the disc of iron be moved with a velocity 

 continually increasing, it is at first acted upon by the steel plate 

 or piece presented to it ; with a certain velocity, no action either 

 way takes place ; and with a still greater velocity, the steel is cut 

 by the iron. In explanation of this fact, M. Allou states, that if 

 two bodies equally hard and elastic, such as two balls of ivory, 

 strike each other with velocities nearly equal, each of these will 

 experience a similar blow, and the change of form which results 

 is instantly corrected by the elasticity of the bodies. But if the 

 first is moved with a velocity incomparably greater than the 

 second, and if the latter, though harder than the former, is suscep- 

 tible of being cut or traversed, it will suffer either the one or the 

 other effect, without receiving any sensible velocity, all the action 

 impressed upon it being employed in piercing or penetrating the 

 ball or body struck. 



2D2 



