1 44 Royal Society. 



this exhaustion is consistent with health. The law of excitement of 

 the muscular fibre, with which both the vital and sensitive parts of the 

 brain and spinal marrow are associated, namely, the muscles of respi- 

 ration, is interrupted excitement, which, like the excitement of the 

 vital parts of these organs, is, only when excessive, followed by any 

 degree of exhaustion. The author conceives that the nature of the 

 muscular fibre is everywhere the same j the apparent differences in 

 the nature of the muscles of voluntary and involuntary motion de- 

 pending on the differences of their functions, and on the circumstances 

 in which they are placed : and he concludes, that, during sleep, the 

 vital, partaking in no degree of the exhaustion of the sensitive system, 

 appears to do so simply in consequence of the influence of the latter 

 on the function of respiration, the only vital function in which these 

 systems cooperate. 



The author proceeds to make some observations on the cause of 

 dreaming, the phenomena of which he conceives to be a natural con- 

 sequence of the preceding proposition. In ordinary sleep, the sen- 

 sitive parts of the brain, with which the powers of the mind are asso- 

 ciated, are not in a state of such complete exhaustion as to preclude 

 their being excited by slight causes of irritation, such as those which 

 accompany the internal processes going on in the system. The sen- 

 sorium is the more sensible to the impressions made by these internal 

 causes, inasmuch as all the avenues to external impressions are closed, 

 and the mind is deprived of the control it exercises, during its waking 

 hours, over the train of its thoughts, by the help of the perceptions 

 derived from the senses, and the employment of words for detaining 

 its ideas, and rendering them objects of steady attention, and subjects 

 of comparison. 



March 14. — A paper was read, entitled, " On the Figures obtained 

 by strewing Sand on Vibrating Surfaces, commonly called Acoustic 

 Figures." By Charles Wheatstone, Esq. Communicated by Michael 

 Faraday, Esq. D.C.L. F.R.S. 



The author, after adverting to the imperfect notice taken by Gali- 

 leo and by Hooke of the phsenomena which form the subject of this 

 paper, ascribes to Chladni exclusively the merit of the discovery of 

 the symmetrical figures exhibited by plates of regular form when 

 made to sound. He proposes a notation, by means of two numbers 

 separated by a vertical line, for expressing the figures resulting from 

 the vibrations of square or rectangular plates. He gives a table of 

 the relative sounds expressed both by their musical names and by the 

 number of their vibrations, of all the modes of vibration of a square 

 plate, as ascertained by the experiments of Chladni. He then pro- 

 ceeds to class and analyse the various phenomena observed under 

 these circumstances, and shows that all the figures of these vibrating 

 surfaces are the resultants of very simple modes of oscillation, occur- 

 ring isochronously, and superposed upon one another ; the resultant 

 figure varying with the component modes of the vibration, the num- 

 ber of the superpositions, and the angles at which they are superposed. 

 In the present paper, which forms the first part of his investigation, he 

 confines himself to the figures of square and other rectangular plates. 



