VOL. XXII. J PHILOSOPHICAL TRANSACTIONS. 49I 



In like manner this n)ay be accommodated to the spherical sector, by drawing 

 i-R, a third part of the common altitude of all the pyramids in it, into a portion 

 of the spherical surface cut off by a plane : which is to the whole spherical sur- 

 face, as the part cut off of the diameter or axis, is to the whole diameter ; as 

 was shown above. 



Now the reason of this whole process depends on these principles. That a 

 figure composed of triangles is half the figure composed of parallelograms, upon 

 the same bases and of equal height. That I call a convolute figure, and this an 

 evolute. And that a figure of pyramids is a third part of a figure of paralielo- 

 pipeds, on the same bases and equal height. That I call a complicate figure, 

 and this an explicate. 



These principles may be accommodated in a thousand manners to curvilinear 

 figures, whether superficial or solid, however perplexed and intricate. 



Concerning the Circulation and Globules of the Blood in Butts. By M. Leuiven- 



hoech. N° 263, p. 552. 



Butts * are small, some of them, bating the tail, not above an inch in length. 

 Having often examined them with a magnifying-glass, in order to see the cir- 

 culation of the blood, and the variety of its motion, my thoughts were again 

 turned to those little particles which constitute the blood of a red colour, which 

 I formerly asserted to be flat and oval. 



The greatest motion of the blood observable through the fins, was on each 

 side of the various little single bones placed among them ; where the blood 

 vessels were so large that 25 of the above-mentioned particles could pass a-breast; 

 but disappeared as they drew nearer to the extremity of the fins, small vessels 

 being all along detached from the arteries. On one side of a little bone runs an 

 artery, and on the other a vein corresponding to it ; and besides a vein and an 

 artery lay so close together, as if their coats had been united. From the above- 

 mentioned artery, there arose smaller vessels across the membrane between the 

 little bones, and after running out the breadth of three or four hairs, they unite 

 again into one vein. These small vessels receive about two or three particles of 

 the blood at once, especially if the fish lies still, and consequently its little bones 

 close to each other. When it exercises its fins in swimming, the distance be- 

 tween the little bones is enlarged, the interjacent membranes stretched out, and 

 the blood vessels that run across them, especially those in the tail- fin, are drawn 

 out so as to be above twice as long as before. 



Finding it very easy to extend the tail-fin, and accordingly having stretched 



* The fry of plaice and of flounders. 

 3 R 2 



