Standard Meafuyt. — ABlon of the pfin^s of a Fly. 35 



afcertain a ftandard of weight and meafure, did not find any wire which would fu'pport the 

 weight of the pendulum in this machine during any confiderable period of time, while it was in 

 his pofleflion in 1796, though he ufed wire nearly twice as think as that of Mr. Whitehurft, 

 From this incident, I fhould have fufpeded that Mr. Whitehurft himfelf might have annealed 

 his wire to a certain point, as he faid it was tempered, if it did not appear from a pafTage in Dr. 

 Fordyce's paper, that he himfelf ufed it from the bobbin. For, he fays, that the curvature was 

 not entirely unfolded for fome months, as he concludes from the clock having loft in its rate 

 during that time. 



Upon the whole, then, though the apparatus for making experiments with pendulums is 

 brought to a confiderable degree of perfedtion, yet it appears that the fundamental experiments 

 ftill remain to be made, in order to afford a ftandard meafure ; the old refult of Whitehurft 

 being the only one we are, at prefent, in pofleflion of. 



VIII. 



On the Vihration of the TVings of a Fly. By S. R. 



To Mr. NICHOLSON. 

 SIR, 



X H E connection of the different departments of fcience with each other is often fliewn in 

 refearches where it might leaft be fufpecled, and it frequently happens, that fubjeds of no ap- 

 parent or immediate utility may give birth to reflexions which afford an innocent and intereft- 

 ing fource of entertainment, and often lead to more valuable confequences. Some time, in the 

 courfe of laft fummer, my thoughts were accidentally diredted to the folution of the problem of 

 the frequency with which flies and other fmall creatures move their wings. If we were to 

 examine the fads upon the general principles of mechanics, taking for our data the fize and 

 inclination of the wing, and the velocity of flight, we fliould certainly obtain a very rapid 

 feries of ftrokes; but there are two other methods of direct experiment, the firft deduced from 

 the dodtrine of found, and the latter from optical principles, which give an immediate refult. 

 As the number of vibrations performed by a fonorous body in a fecond, to produce a given 

 note are known, and no infedl has any other voice than that which is afforded by the immedi- 

 ate vibration of its wings or other mechanical adtion, we may eftimate the frequency of vibra- 

 tion in the bee, the fly, and the gnat, from the refpedlive notes with which their tones are in 

 unifon. But, into this enquiry I fliall not enter, becaufe the other optical method has anfwered 

 my purpofe. 



M. D' Arcy *, Dr. Watfon f, and others, have (hewn that the imprefllon of light upon the 

 eye endures for a time after the objedt has ceafed to exert its adtion, and this truth is familiarly 

 illuftrated by the common experiment of whirling the end of a lighted ftick ; the ignited extre- 

 mity of which will appear to form a circle, if the motion be quick enough to bring the end 

 round to its firft place, before the original impreflion has ceafed. When a common fly moves 

 horizontally towards the fun, and the obferver views him in a line at right angles to his flight 



* Memoirs of the Paris Academy, 1765. f On time, 



F 2 (both 



