PHYSICS. 95 



placed in melted metal at first sink, but in a few seconds 

 rise again and float on the surface. Flat bars of cast iron 

 carefully laid on the surface continue to float. A solid ball 

 2f inches in diameter, lowered into the metal by a fine wire, 

 disappeared completely at first, but rose in a few seconds 

 and floated, with about half an inch diameter of surface ex- 

 posed. Since in foundry practice ^ is allowed for linear 

 contraction of cast iron, the author believes that the finally 

 cooled solid is denser than the molten metal ; but as the 

 sharpness of iron castings points to an expansion on solidifi- 

 cation, he also believes that the contraction in cooling more 

 than counterbalances the expansion during solidification. 

 This view of the case is fully supported by the experiments 

 on floating above described. 



Sire has devised a new form of apparatus for demonstrat- 

 ing the hydrostatic paradox of Pascal. It consists as usual 

 of three containing vessels, one cylindrical, the other two 

 conical, the first with its base upward, the second with the 

 base downward ; but in the new apparatus the three are 

 cemented at bottom into rings, giving their bases absolutely 

 the same area. Below these rings are three glass cylinders 

 communicating with each other, and filled with mercury. 

 On filling the vessels with water, and opening communica- 

 tion between them to equalize the level in them all, the 

 mercury in all the cylinders below is observed to stand at 

 exactly the same height. 



Hasler has proposed a new water meter, the peculiarity of 

 which consists in the mode of counting. Upon the axis of 

 the revolving drum is a steel bar magnet, which revolves 

 close to the partition separating it from the counting wheels. 

 Upon the axis of the lowest of these wheels is a second mag- 

 netized steel bar, smaller than the former. This is carried 

 round by the larger bar solely through its magnetic attrac- 

 tion, and so effects the registration. 



Trowbridge has made a series of ingenious experiments 

 on vortex-rings in liquids, analogous to the smoke-rings of 

 Thomson and Tait. Applying to this case the general equa- 

 tions of vortex motion, he draws the conclusion not onlv 

 that all liquids falling upon the free surface of liquids from 

 such a height that the surface of the liquid is not too much 

 disturbed to enable the drop to be acted upon symmetric- 



