VOL. XXIX.] PHILOSOPHICAL TRANSACTIONS. iQj 



Prop. Proh. — To find the Distance of the Centre of Percussion from the Plane 

 passing through the Centre of Gravity, and perpendicular to the Axis of Rotation. 

 — Solution. Let fig. 12, pi. 4, be supposed in the plane passing through the 

 axis of rotation, and in which the centre of perscussion is sought. Let ab be 

 the axis of rotation; agc the intersection of this figure with the plane passing 

 through the centre of gravity, and perpendicular to the axis of rotation; 

 G the point on which a line, raised perpendicular to this figure, will pass 

 through the centre of gravity; be a line parallel to ag, which is the centre 

 of percussion. Then to find the distance ab, let p stand for an element of 

 the body proposed standing perpendicularly on any point d. Draw dc 

 perpendicular to agc; then ab will be equal to the sum of all the quan- 

 tities p X Gc X CD taken with their proper signs, divided by the body itself 

 multiplied into the distance ag. 



Having thus found the distance ab, suppose the plane of the figure in 

 Prop. 15 to cut the present figure at right angles in the line be, and the centre 

 of percussion will be rightly determined by that Proposition. 



The 26th Proposition shows how to determine the density of the air at any 

 distance from the centre of the earth, supposing the density always to be pro- 

 portional to the compressing force, and that the power of gravitation is reci- 

 procally at the distances from the centre of the earth. 



The last Proposition shows how to find the refraction of a ray of light in its 

 passage through the atmosphere, on the supposition that light is a body, and 

 that its refraction is caused by the attraction of the bodies the rays approach to. 

 In this proposition there is a remarkable instance of the usefulness of the 

 method of increments, in finding the co-efficients of a Series, which according . 

 to the values of a certain symbol, as 77, expresses both all the fluents, and all 

 the fluxions of a certain quantity. 



//. Ludovici Ferdinandi Marsilii Disseriatio de Generatione Fungorum. Rom. 



1714, Ato. W 345, p. 350. 



In this dissertation the author gives an account of the various opinions, 

 both ancient and modern, respecting the generation of mushrooms. He sup- 

 poses the seed-like bodies observable in the fungus seminifer campaniformis 

 Mentzelii, to be the ovaria of some insects ; and therefore he concludes that 

 these bodies ought to have another denomination than seed ; neither is he of 

 opinion that mushrooms are produced by parts of themselves. In his division 

 of mushrooms he 1st treats of truffles; 2dly, of those mushrooms (fungi) 

 which grow from wood, but are soft; Sdly, of hard woody mushrooms. Of 



c c 2 



