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Dr. Barrow to Mr. Willughby. 



Mojl Honoured Sir, tfrm Col. 051. j\ \66f m 



YOUR Difcourfc inferring the Solidity of the 

 Sphere from the Surface., by comparing the 

 Concentrical Surfaces of the Sphere with the Parallel 

 Circles of the Cone, is very ingenious and folid. I did 

 formerly ufe fomewhat a like Method, thus : 



Dividing the Radius in- 

 to equal Parts indefinitely, 

 and fupofing equal Sur- 

 faces concentrical through 

 the Divifions, they will 

 proceed increafing as the 

 Squares of their Radii ('tis 

 the Property of all like Sur- 

 faces) that is as i, 4,9, &c. 

 Wherefore if the greateft 

 be S, the Sum of all will 

 be rS = Sphere, and if S =4 greater Circles — zrp* 



the Sphere will be -~ rrp. but rrp is a ■ Cylindre, 

 whofe Bails rp, altitude zr. By the fame Method 



2 



you may find the Solidity of a Spherical Se&or, fup- 

 pofing its Spherical Superficies known, and conse- 

 quently of the Portion it contains. 



y. eft Radius. 

 f , eft Pcriphcda, 



Your 



