PHYSICS. 461 



placed by one of 127, and the pitch will be one millimeter. {Am. J. ScL, 

 March, 1882, III, xxm, p. 176.) 



Mascart has experimented with a new form of apparatus devised to 

 determine variations in the intensity of gravity at different places by 

 noting the height of the mercury column which balances the elasticity 

 of the same mass of gas at a constant temperature. It resembles a 

 siphon barometer, but the shorter leg is closed, confining above the mer- 

 cury within it, carbon dioxide gas. The longer leg is open and contains 

 a meter column of mercury. The author believes it will compare favor- 

 ably in sensibility with the pendulum. {Comptes Bendus, July, xcv, p. 

 126.) In a subsequent communication, Mascart gives the results of some 

 measurements taken with this gravity barometer at various places in 

 the north of Europe, confirming his previous opinion. ( Gomptes Rendus^ 

 October, xcv, p. 631.) 



An ingenious deep-sea sounding apparatus has been devised by a 

 Russian naval officer. It consists of a piece of lead, a small wheel with 

 a contrivance for registering the number of revolutions, and a float. As 

 the apparatus sinks, the wheel revolves and registers the depth. On 

 reaching the bottom the lead is detached and the float brings the rest 

 of the instrument to the surface. {Nature, March, 1882, xxv, p. 471.) 



MECHANICS. 



•1. Of Solids. 



Greenhill has determined mathematically the greatest height con- 

 sistent with stability that a vertical pole or mast can be made, and the 

 greatest height to which a tree of given proportions can grow. He finds, 

 for example, that 90 meters is the maximum height for a pine tree half 

 a meter in diameter at the base. {J. Fhys., July, II, i, p. 337.) 



Colsou has observed that the increase of carbon absorbed by a new 

 iron disk when heated in charcoal over one already partially carburized, 

 is only apparent, being due to the reaction of the iron upon the carbon- 

 ous oxide. Hence, on heating, oxygen is evolved. For a given temper- 

 ature, therefore, he maintains that the coefficient of diffusion of carbon 

 into iron is constant ; at least until the limit for steel has passed. Silicium 

 diffuses with great ease also. A platinum wire, immersed in lampblack 

 which was proved to be free from silica, placed in an earthen crucible, 

 was found to be silicified after the whole had been heated in a forge 

 fire; and the lampblack, on burning, left a residue of white silica. On 

 longer heating, the platinum becomes brittle and acquires the composi- 

 tion Si Pt2. If silica be mixed with the lampblack a still higher com- 

 pound, Si2 Pts, results, having a density of 14.1, and fusing at the same 

 temperature as ordinary glass. The author believes that the silica dif- 

 fuses as such and not as silicium. Iron heated with a mixture of silica 

 and carbon gives a crystalline silicide of density 6.6, containing 15 per 

 cent, of silicium. Colson considers that silicium may be gaseous ; as 



