INTRODUCTION. 



THIS Essay contains nothing that will be new to 

 philosophical readers; for it is little more than 

 the substance of two Papers which have already ap- 

 peared in the Philosophical Transactions of the Royal 

 Society of London; one in the year 1786, and the 

 other (for which the Author had the honour to receive 

 from the Society the Copleian Annual Medal) in the 

 year 1792. 



As reference has frequently been made to these Pa- 

 pers in several of the preceding Essays ; and as many 

 of the experiments of which an account is given in them 

 are not only interesting in themselves, but are necessary 

 to be known in all their details in order to judge of 

 several important conclusions that have been founded 

 on their results, the Author has thought that it would 

 not be improper to republish them under the present 

 form. He was also desirous of adding the substance 

 of those Papers to his Sixth and Seventh Essays, in 

 order that all that he has written on the Science of Heat 

 might be brought together in one volume. 



The Essays which are destined to compose the next 

 volume (many of which are already in great forward- 

 ness) are all on practical subjects of a popular nature, 

 and of general utility ; and on that account it was 

 judged best to keep them separate from those contained 



