348 On the Forms and States of Fluids [1831. 



quite free from mark, and fully distended. All these are na- 

 tural consequences, if the film be considered as a flexible but 

 inelastic envelope formed over the whole surface whilst the 

 heaps were rising and falling. 



103. The mode of action by which these heaps are formed 

 is now very evident, and is analogous in some points to that by 

 which the currents and the involving heaps already described 

 are produced. The plate in rising tends to lift the overlying 

 fluid, and in falling to recede from it ; and the force which it 

 is competent to communicate to the fluid can, in consequence 

 of the physical qualities of the latter, be transferred from par- 

 ticle to particle in any direction. The heaps are at their maxi- 

 mum elevation just after the plate begins to recede from them ; 

 before it has completed its motion downwards, the pressure of 

 the atmosphere and that part of the force of the plate which 

 through cohesion is communicated to them, has acted, and by the 

 time the plate has begun to return, it meets them endowed with 

 momentum in the opposite direction, in consequence of which 

 they do not rise as a heap, but expand laterally, all the forces 

 in action combining to raise a similar set of heaps, at exactly 

 intermediate distances, which attain their maximum height just 

 after the plate again begins to recede ; these therefore undergo 

 a similar process of demolition, being resolved into exact dupli- 

 cates of the first heaps. Thus the two sets oscillate with each 

 vibration of the plate, and the action is sustained so long as 

 the plate moves with a certain degree of force ; much of that 

 force being occupied in sustaining this oscillation of the fluid 

 against the resistance offered by the cohesion of the fluid, the 

 air, the friction on the plate, and other causes. Fig. 26. 



104. A natural reason now appears for the 

 quadrangular and right-angled arrangement 

 which is assumed, when the crispation is most 

 perfect. The hexagon, the square, and the 

 equilateral triangle are the only regular figures 

 that can fill an area perfectly. The square and 

 triangle are the only figures that can allow of 

 one half alternating symmetrically with the 

 other, in conformity with what takes place be- 

 tween the two reciprocating sets of heaps, 

 fig. 26 ; and of these two the boundary lines 



