292 



Extremes of 

 frequency. 



Trembling of 

 ag<: or weak- 

 ness. 



The obserra- 

 tion not new. 



but the f»ct 

 ascfibed to a 

 ^viar.g cause. 



Sound of car- 

 riages at a »i;=; 



DURATION OF MUSCULAR ACTION. 



cording to the degree of force exerted in tbe experi- 

 ment*. 



As a farther proof, that I was not much deceived in my 

 judgment of the frequency of these vibrations, I requested 

 two or three of my friends to repeat the same experiment 

 for me, and our agreement was such, as to confirm me in 

 the opinion, that there could be no very considerable errour 

 in the estimate. 



The greatest frequency, that I think I have observed, was 

 about 35 or 36 in a second, and the least was as low as 14 

 or 15 ; but in attempting to lessen the number of vibra- 

 tions, there appears to be a degree of unsteadiness, which 

 prevents any accurate measurement of the real number. 



It is very probable, that in cases of great debility the 

 numtier may be even considerably less, and may be the 

 reason of that visible unsteadiness, which is known to occur 

 in persons enfeebled by age, or much reduced by disease. 



Possibly the foregoing observation may not be altogether 

 new to some members of this Society, as it is now about 17 

 or 18 years since it first occurred to me, and I was then ac- 

 customed occasionally to mention it in conversation with my 

 friends ; but I am not aware that any other person has made 

 the same remark respecting the vibratory nature of mus- 

 cular action, although I find, that Grimaldi had observed 

 the sound that occurs upon stopping the ears, but ascribed 

 it, according to the notions that prevailed in his time, to the 

 hurried motions of the animal spirits f. 



* The resemblance of the muscular vibrations to the sound of car- 

 riages at * distance 1 apprehend to arise not so much from the quality of 

 the sound, as from an agreement in frequency wiili an average of the 

 tremor? usually produced by the number of stones in the regular pave- 

 inent'of London, passed over by carriages moving quickly. 

 . 'If the number of vibrations be supposed 24 in a second, and the 

 hreadih of each stone be about 6 inches, the rate of a carriage thus esii- 

 mated virould be about 8 miles an hour, which agrees with the truth as 

 nearly as tlie assumptions on which the estimate is founded. 



■f Vera itaque ratio experimeuti pr«dicti est, quia in digtto et brachio 

 totuque coipore co.'itinuato (iunt multi motus ac tremores, ob spirituum 

 ?glr,ri';o:)cm h'.icilluc perpetuoaccurrentiiim. 



».)n;-!Ai.Di, Physicomathcsis (1« Limine, p. 383. 



Part II. 



