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are hollow within, and, if hair and wool, (which is the hair of fheep) 

 were not compofed of many minute hairs or fibres, they would not 

 have that ftrength and toughnels which we obferve in them. 



Sometimes, in human hairs, and efpecially in the very middle 

 of them, I have obferved a dark line : particularly in feveral of the 

 hairs taken from my own beard: and when I attentively exa- 

 mined this dark line, I found it to confift of fuch minute and flender 

 particles, as to be almoft undifcoverable even by the microfcope. 

 Examining fome very fmall hairs, of three, four or five days 

 growtli, and finding fome to be throughout quite tranfparent, others 

 darkened only in a very fmall degree, and finally, others with no 

 more than a fmall dark i'pot on them, I began to confider, whether 

 this dark Ihade in tlie hairs, might not proceed from fome particles 

 of blood in the fabftance of the hair, and there dried. 



To give the reader an idea of tliis dark line, I caufed a drawing to 

 be made of a piece of a fingle hair, which I concluded to be of liiree 

 days growth ; this is to be feen 'dtfg. 14,, O P()RSTV W, in which 

 at (^RS and V WO, are the two ends where tiie piece of hair was 

 cut with a knife : from W to P, or as far as T, the dark line I have 

 mentioned extends, wiiich I have mentioned to be vifible in fome 

 hairs, and in others not fo confpicuous. Laftly, between R and 

 T P are reprefented thofe dark fpots, which are to be obferved in 

 other hairs. 





