Parsons. — On a Reflecting Telescope made in Wellington. 125 



S ; and because a uniformly accelerating 

 force is measured by twice the space 

 described from rest in one second, and 

 it is found by experiment that the force 

 of gravity on the earth's surface causes 

 a body to fall from rest a distance of 

 16*1 feet in the first second of time, 

 consequently the force of terrestial 

 gravity g — 32 • 2 feet, that is, this 

 force continuously soliciting a falling 

 Fig. 5. body, will accelerate its velocity 32*2 



feet every second ; therefore 2AE expresses the intensity of gravity acting 

 on A. Join BA ; then since the arc AB differs insensibly from its chord 

 (for the time of describing it may be made as small as we please) we may 

 regard ABA' as a right angled plane triangle since the angle B is in a 

 semicircle, therefore AE : AB : : AB : A A'. 



AA' "" 2AS • ' ""AS ~ r 



Now 2AE represents the accelerating force at S, or taken in an opposite 

 direction, it represents the centrifugal force f, and AB represents the velocity 



V in the curve ; consequently the centrifugal force y* = — , where r = radius. 



If, as is usual, n be made to stand for the number 3*14159, etc., the whole 

 circumference of the circle will be 2nr ; therefore calling the whole time of 

 describing the circumference — that is the periodic time, t — then the uniform 

 velocity v being equal to the whole space divided by the whole time we have- — 



2 71 r ^ 4c n^ r 



. .- 2nr . t)^ 2 nr 2nr 4:n^ r 



for II v= then — = x r- r: 



t r t t ' t^ 



Deviation from the parabolic figure arising from the earth's sphericity only 

 amounts to g^jgoo ^^ ^^ iTLch. at the circumference of a speculum four feet in 

 diameter. 



Art. XII. — Description of a Reflecting Telescope made in Wellington hy 

 W. F, Parsons. Communicated by James Hector, M.D., F.B.S. 



[Read before the Wellington Philosophical Society, 23rd October, 1872.] 



The instrument which I exhibit is a Newtonian model, with a silvered -glass 

 speculum, and with the exception of the eye-pieces has been wholly made in 

 Wellington by following the directions given in a paper by Mr. W. Purkies, 



