234 FHILOSOPHICAL TRANSACTIONS. [aNNO 1728. 



greater force than at present it can be; because the internal coat, being com- 

 posed of annular fasciculi, whose sides have but a very weak cohesion, their 

 power of resisting will not be measurable by the strength of those annuli ; but 

 by the force with which they adhere laterally. And on the other hand, the 

 external coat, being composed of fibres equally interwoven, and of a quite dif- 

 ferent composition, it may either exert a greater resistance, or be capable of 

 much greater dilatations than the internal. 



But that autopsy may evince the truth of this difference in the strength of 

 these coats, it will be found by any one who pleases to try the experiment, that 

 by blowing into the pulmonary artery, the internal coat will soon burst, and the 

 external form itself into aneurismous tumours, as was proved by experiments. 



On considering all which, and having, by order of the Society, both privately 

 and publicly examined the aneurism before them, which Mr. N. finds tobe round 

 like other extravasate tumours, unless when controlled by any notable pressure, 

 and that the sacciilus does not divide into coats, as the artery from whence it 

 arises does; he is induced to think that this aneurism is a tumour formed by the 

 blood being forced through the ligamentous, or what is called the muscular 

 coat, and distending the membranous or outer one. And because the impetus 

 of the blood will, as it were, perpetually press through the aperture into the 

 tumour, and be again in part returned by the elasticity of the external coat; 

 therefore such a tumour will rather have a pulsatile dilatation, than a pulsation, 

 for its true diagnostic. 



Of a surprising Shoal of Pumice-Stones found floating on the Sea. By Mr. John 

 Dove. N° 402, p. 444. 



On the 22d of March, 172^, at noon, being in the latitude 35° 36' south, 

 and longitude 4° q' west, with variation 3° l6' w. they discovered several pumice- 

 stones on the sea; but not expecting any such thing at that distance from the 

 land, the islands Tristan d'Acunha being the nearest, which were judged to 

 bear w. 9° lO' s. distance 186 leagues, they disputed what it might be; 

 when about 1 p. m. they took up a piece in a backet, which confirmed Mr. 

 Dove's opinion of its being pumice-stones. Towards night it was spread all 

 round, as far as could be seen. Next morning the pumice-stones were very 

 thick, in drifts, lying n. n. e. and s. s. w^. and extended out of sight from the 

 mast's head, increasing as they ran to the eastward. 



Wednesday the 24th they continued their course e. s. e. 140 miles, the 

 pumice-stones being thicker; so that for 16 hours some of the drifts were about 

 a cable's length broad, and so thick, they could scarcely see the water between 

 them ; and there was much the same breadth between the drifts, with several 



