NATURAL PHILOSOPHY. 155 



experiment, viz., by placing behind it a platina wire kept glowing 

 by a galvanic battery. The continuous part of the wire was hidden ; 

 but the portion behind the wavering end, he said, became separated into 

 dots of light and spaces hidden by the drops. He also showed that 

 lateral motions of bodies in water, when rapid enough, caused both 

 bubbles and sounds ; and he accounted for their production in both 

 cases by the surface closing over the pit formed by the descending 

 drops or laterally moving body ; the inclosed air is then carried for- 

 ward, and at length, ascending to the surface, bursts with the explosion 

 which causes the sounds. 



A writer in the Philosophical Magazine, in this connection, makes 

 the following remarks on the origin of the sound of agitated water. 

 " When the smoke is projected from the lips of a tobacco-smoker, a 

 little explosion usually accompanies the puff ; but the nature of this is 

 in a great measure dependent on the state of the lips at the time, 

 whether they be dry or moist. The sound appears to be chiefly due to 

 the sudden bursting of the film which connects both lips. If an inflated 

 bladder be jumped upon, it will emit an explosion as loud as a pistol- 

 shot. Sound, to some extent, always accompanies the sudden libera- 

 tion of compressed air. If the surface of the fluid on which a jet falls 

 intersects its limpid portion, the jet enters silently, and no bubbles, as 

 before remarked, are produced. The moment, however, after the 

 bubbles make their appearance, an audible rattle also commences, 

 which becomes louder and louder as the mass of the jet is increased. 

 The very nature of the sound pronounces its origin to be the bursting 

 of the bubbles ; and to the same cause the rippling of streams and the 

 sound of breakers appear to be almost exclusively due. I have exam- 

 ined a stream or two, and, in all cases where a ripple made itself heard, 

 I have discovered bubbles. The impact of water against water is a 

 comparatively subordinate cause, and could never of itself occasion the 

 murmur of a brook or the musical roar of the ocean. It is the same as 

 regards water-falls. Were Niagara continuous, and without lateral 

 vibration, it would be as silent as a cataract of ice. It is possible, I 

 believe, to get behind the descending water at one place, and if the 

 attention of travellers were directed to the subject, the mass might 

 perhaps be seen through ; for, in all probability, it also has its ' con- 

 tracted sections,' after passing which it is broken into detached 

 masses, which, plunging successively upon the air-bladders formed by 

 their precursors, suddenly liberate their contents, and thus create the 

 thunder of the water-fall. 



FOUCAULT'S EXPERIMENT OK, THE ROTATION OF THE EARTH DEMON- 

 STRATED BY THE PENDULUM. 



FEW incidents in the scientific history of the year 1851 have excited 

 a more general interest than the experiment, devised by M. Leon 

 Foucault, of Paris, for demonstrating the rotation of the earth by means 

 of the pendulum. Although the rotation of the earth has, by demon- 

 stration, been rendered self-evident to all who are in the least degree 

 conversant with the principles of natural philosophy ; yet, by the experi- 



