^^(3 Optical nemmencn txplnimd. 



other. In mod of thefe contrivances, the pendulum or balance is, during fome part of its- 

 motion, brought in the way of one of the teeth of the bit wheel of the train, at the timft' 

 ^yhen the whole ffftcm of wheels are at liberty to move. The tooth, therefore, ftrikes 

 either the pendulum, or a part of the movement conneaed with it, and then flips off, or 

 efcapes from that obftacle. The whole fet of parts in a time-piece by which this alter- 

 na^e^gion.ijfcfijeclcd, is for lliat reafon. called the efcapement. --^=^__=x: 



[To be continiici!.] 



Jl^lAT HEMATIC AL a,\d PHILOSOPHICAL CORRESPONDENCS: ' 



AxswER.To.'KiEilnu.osoriUCAL Question, p. 284. 



Ta liir.^ICHOLSOX. 

 SIR, ' '■•'" '"" 



JL ROM the defcription of the divergent rays feen in agitated water furrounding the 

 fliadow of the obfcrver's head, which forms part of the queftion in your fixth number, 1 did • 

 not fail to look for the appearance on tJic full opportunity of funlhine which ofliered. I 

 obferved the fad as you dcfcribc it, and found that the radiations appear M-ithout being 

 modified in any refpcift by the diftance of the obferver from the furface of the water. Thi?: 

 circumllance indicated a probability, that the caufe which I could not invelligate by direft' 

 reafoning, might be developed by a nearer infpeftion than could with convenience be made' 

 on the bank of a river. With this view I placed a veflel of water, which was three feet deep, 

 in an out-houfe, in fuch a manner that the fun (hone upon its furface. When I placeef' 

 myfelf fo that the (hadow of my head fell upon the water, I perceived no radiations what- ' 

 ever ; but upon agitating the water with my hand, the water appeared full of the lines callet! 

 fun-beams, parallel to each other no doubt, but which, on account of the fmaller optical' 

 diftance between their remoter than between their nearer extremities, had the appearanc* 

 of convergence towards the fliadovv. As it is pcrfeftly intelligible from the well-known 

 truth in pctfpe£live, that all lines parallel to the line of fight Avill v^nifli in the point of' 

 fight ; it remains only to be (hewn how thefe diflinft parallel beams are produced. ' 



When the furface of the water is at reft, and forms one plane, the fun's light fonns one 

 uniform mafs in which no lines arc diilinguifhable; but when the agitation takei place, a 

 number of convexities and concavities are produced. Thefe fcatter the rays in all di- 

 rc£lions, except at that fmall portion where the contrary curvatures join. The fame 

 phyfical effedt is produced as from fo many fmall planes ; that is to fay, the r.iys pafs 

 flraight through eaoh furface^and afFord the parallel diftini^ beams of light. And though 

 thefe fmall planes are continually forming and difappearing, yet there are always a certain 

 number in cxiftence, and the eft'cdt of their trarifient individual variations is nothing more 

 than to produce an undulation in the general fyflem of radiations. ' • 



And now. Sir, after having refoved your queftion, as I think, fatis^iclorlly ; I (h&ll take 

 the liberty to prOpofe another ili the fame department of fciencc. 



When the fun fliine?, if two bodies, for example hands or fingers, be gradually moved, 

 fo that their Ihadowj mty «>eet j it is obfcnfcd that the (hadow of the body fartheft from 



the 



