Mr. Potter on giving Conic-sectional Figures to Lenses, S^c. 55 



Force required to crush per square Inch. 



lbs. 



P. 337. A li-inch brick pier, laid in cement A 470 



Pozzolano, 3 parts; ground lime, 1... 296 



Atkinson's cement, 1 ; sand, 1 410 



Pozzolano, 4; lime, 1 638 



Ditto, 3; Dorking lime, 1 600 



Stone-lime, 1 ; sand, 3 500 



Portland-stone pier 2300 



A small error may be corrected, Phil. Mag. and Annals, 

 vol. xi. page 339, line 20,— ^or 173^ tons, read 149 tons. 



Yours truly, 



B. Bevan. 



P.S. — From the disproportion between the cohesive strength 

 of pure cement, and cement used in brickwork, it is desirable 

 that further experiments should be made on this subject. 



XIV. Addendtim to the Paper on a Method for giving the Fi- 

 gures of the Conic Sections to Concave Lenses and Specula, 

 published in No. XII. of the Edinburgh Journal of Science. 

 By R. Potter, Fsq. Jun.* 



TITAVING lately had occasion to look into the Transactions 

 -*--*• of the Cambridge Pliilosophical Society, I fell upon a pa- 

 per read before the Society on the 1 1 th December, ] 822, by the 

 Rev. W, Cecil, M.A. of Magdalen College, " On an apparatus 

 for grinding telescopic mirrors and object lenses;" where the 

 author, though his principal object appears to be that of de- 

 scribing a machine for superseding manual labour in the ge- 

 neral workmanship, shows from mathematical principles the 

 quantity which should be worn away at the various points, to 

 bring a concave lens or speculun,! to the figure of a conic sec- 

 tion. Thus far it appeal's that I am completely anticipated; 

 but as we differ in our directions for reducing the theory to 

 practice, I submit my claim to the effectual invention to the 

 judgement of the scientific world. 



Mr. Cecil does not refer to any examination of specula, &c. 

 worked according to his directions. I did not pubhsh my 

 discovery until I had lully proved it, practically, in such large 

 proportions of specula as left no doubt of my procedure being 

 correct. I Iiave prescribed the rotatory effect in the lathe, alone, 

 to be used, even iti the Jiiiishing process. Mr. Cecil prescribes a 

 small lateral motion as well as a circular one, saying, " to grind 



* ComiiiunicatctI bv the Aiilhor. 



