VI PREFACE. 



the several subjects; with references to such passages as appeared to 

 be most important: it was therefore necessary, as well for this purpose, 

 as in order to procure all possible information that could tend to the 

 improvement of the work, to look over a select library of books en- 

 tirely with this view, making notes of the principal subjects discussed 

 in them, and examining carefully such parts as appeared to deserve 

 more than ordinary attention. Hence arose a catalogue of references ; 

 respecting which it is sufficient to say, that the labour of arranging 

 about twenty thousand articles, in a systematic form, was by no means 

 less considerable than that of collecting them. The transactions of 

 scientific societies, and the best and latest periodical publications, 

 which have so much multiplied the number of the sources of informa- 

 tion, constituted no small part of the collection, which was thus to be 

 reduced into one body of science. 



' With the addition of the materials acquired in making this compi- 

 lation, and of the results of many original investigations, to which 

 they had given rise, it became almost indispensable to copy the 

 whole of the lectures once more, and to exchange some of them for 

 others, which were wholly new; at the same time all possible pains 

 were taken to discover and to correct every obscurity of expression or 

 of argument. Drawings were also to be made, for representing to the 

 reader the apparatus and experiments exhibited at the time of deliver- 

 ing the lectures, for showing the construction of a variety of machines 

 and instruments connected with the different subjects to be explained, 

 and for illustrating them in many other ways. These figures have been 

 extended to more than forty plates, very closely engraved, and the 

 execution of the engravings has been minutely superintended. But 

 the text of the lectures has been made so independent of the figures, 

 that the reader is never interrupted in the middle of a chain of reason- 

 ing, but is referred, at the end of a paragraph, to a plate, which has 

 always a sufficient explanation on the opposite page. . >: 



