PREFACE. 



THE following sheets comprise the remaining part of the 

 discoveries and experiments of my late friend Mr. William 

 Hewson, in whose death the public sustained an almost irre- 

 parable loss ; the loss of a genius, whose superior abilities in 

 his profession rendered him eminently conspicuous among his 

 cotemporaries, and I have no doubt will transmit his fame to 

 posterity, enrolled among the highest names of antiquity. But 

 to the men of science of this age, his talents require no com- 

 mendation from my pen. 



Unfortunately for the world, his death prevented him from 

 completing the work he had begun : the first chapter of this 

 treatise only was written by him, and read in the Royal Society, 

 June 17th and 24th, 1773; which was afterwards published in 

 the second part of the sixty-third volume of the Philosophical 

 Transactions ; and it is much to be lamented, that among his 

 papers we have not found the smallest note upon the subject 

 of the other four chapters. But a three years' acquaintance, 

 during the greatest part of which the strictest intimacy and 

 friendship subsisted between us, afforded me numberless oppor- 

 ttmities of discoursing with him upon this subject, and of 

 making myself perfectly acquainted with his ideas ; besides 

 which, as I had the advantage of assisting him in other anato- 

 mical pursuits, it was frequently my good fortune to make and 

 repeat many of the experiments; by which means I became 

 not only better acquainted with the doctrine, but also perfectly 

 confirmed in my knowledge of its truth. 



As far as I can recollect, I have recited the experiments in 

 the order they were made by Mr. Hewson ; but lest I might 

 err, or not represent facts in their true state, I have repeated 

 all the experiments frequently since his death, and have written 

 them circumstantially as they appeared to me. But to make 



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