COUNT KUMFOED. 165 



As already mentioned, the fact of water changing 

 its density at a temperature of 40 Fahr. powerfully 

 affected the mind of Rumford. On this subject he 

 made many experiments; and one of the minor applica- 

 tions of the knowledge thus derived may be here noted. 

 In company with his friend Professor Pictet, of Greneva, 

 he paid a visit to the Mer de Glace, and discovered 

 in the ice a pit ' perfectly cylindrical, about seven inches 

 in diameter, and more than four feet deep, quite full of 

 water.' He was informed by his guides that these pits 

 are formed in summer, and gradually increase in depth 

 during the warm weather. How can these pits deepen ? 

 Rumford answers thus : The warm winds which in 

 summer blow over the surface of the column of ice-cold 

 water, communicate some small degree of heat to the 

 fluid. The water at the surface being thus rendered 

 specifically heavier, sinks to the bottom of the pit, to 

 which the heat thus carried down is communicated, 

 melting the ice and increasing the depth of the pit. We 

 have here a small specimen of Rumford's penetration, 

 but it is a very interesting one. The sun's invisible 

 rays, however, are probably more influential than the 

 action of the warm wind in producing the observed 

 effect. 



Various interesting experiments were made by 

 Rumford on what is now known as ( surface-tension.' 

 From his experiments he inferred that the surface of a 

 liquid of water, for example is covered by a pellicle 

 which can be caused to tremble throughout by touching 

 it with the point of a needle. He proposed to the 

 geometricians of Paris to determine the shape of a drop 

 resting on a horizontal surface, and restrained solely by 

 the resistance of a pellicle exerting a given force on its 

 surface. This pellicle he considers to be due to the 

 adhesion of the particles of liquids to each other, and he 



