145 



A THI^OREM for /nding the Surface of an OBLiqUE CYLINDER, 

 •with its Geometrical Demonfiration. Alfo, an Appendix, containing fame 

 Obfervations on the Methods of finding the Circumference of a very Eccen- 

 tric Ellipfe ; including a Geometrical Demotijiration of the remarkable Pro- 

 perty of Elliptic Arcs difcovered by Count Fagnani. By the Rev. J. 

 Brinkley, a. M. M. R. I. A. Andrews Frofejfor of AJironomy in 

 the Univerfity of Dublin. 



Read Dec. 20, 1 8oa. 



'R. Barrow in his " Geomefricse Leftlones"* remarks" the difficulty of 

 finding the furfaces of an oblique cylinder, and of an oblique cone, the 

 bafe of which are circles. Mentioning the former, he fpeaks of, " in- 

 " fuperabilis ilia difficultas, quacura confliftantur, qui cylindricas obliquas 

 " fuperficies conantur dimetiri, feu cum cylindricis fuperficiebus reftis aliifve 

 " quadatenus cognitis fuperficiebus quoad proportionem comparare." — Hav- 

 ing obtained a very neat and fimple theorem for the furface of an oblique 

 cylinder, which, if I am not miftaken, is new, I am induced to fub- 

 mit it to the Academy, the more particularly, as it admits a demonfiration 

 purely geometrical. 



* Lea 2. 



Vol. IX. ( T ) « The 



