SECT, xvi.] UNDULATIONS OF THE AIR. 145 



nmnicates its vibrations to the surrounding particles, which 

 transmit them to those adjacent, and so on continually. 

 Hence from the successive vibrations of the particles of air 

 the same regular condensations and rarefactions take place 

 as in the field of corn, producing waves throughout the 

 whole mass of air, though each molecule like each individual 

 ear of corn never moves far from its state of rest. The small 

 waves of a liquid, and the undulations of the air, like waves 

 in the corn, are evidently not real masses moving in the 

 direction in which they are advancing, but merely outlines, 

 motions, or forms passing along, and comprehending all the 

 particles of an undulating fluid which are at once in a 

 vibratory state. It is thus that an impulse given to any 

 one point of the atmosphere is successively propagated in 

 all directions, in a wave diverging as from the centre of a 

 sphere to greater and greater distances, but with decreasing 

 intensity, in consequence of the increasing number of 

 particles of inert matter which the force has to move ; like 

 the waves formed in still water by a falling stone, which are 

 propagated circularly all around the centre of disturbance 

 (N. 156). 



The intensity of sound depends upon the violence and 

 extent of the initial vibrations of air ; but, whatever they 

 may be, each undulation when once formed can only be 

 transmitted straight forwards, and never returns back again 

 unless when rejected by an opposing obstacle. The vibra- 

 tions of the aerial molecules are always extremely small, 

 whereas the waves of sound vary from a few inches to 

 several feet. The various musical instruments, the human 

 voice and that of animals, the singing of birds, the hum of 

 insects, the roar of the cataract, the whistling of the wind, 

 and the other nameless peculiarities of sound, show at once 

 an infinite variety in the modes of aerial vibration, and the 

 astonishing acuteness and delicacy of the ear, thus capable 

 of appreciating the minutest differences in the laws of 

 molecular oscillation. 



