POSTSCRIPT. 



THE design of the last four Treatises has been to offer a condensed and me- 

 thodical work on that important department of Natural Philosophy which com- 

 prises the diversified phenomena of Electricity and Magnetism. These phe- 

 nomena, which were formerly regarded as the effects of two perfectly distinct 

 agents, are now discovered to have an intimate relation to one another, and, 

 in all probability, to be dependent on one and the same principle : in like 

 manner, as it was found by Newton that the simpler mechanical phenomena 

 of the universe are the results of the single principle of gravitation. A suc- 

 cinct and connected account of the numerous discoveries which the exertions 

 of philosophers have recently brought to light on this highly interesting 

 branch of physical science, collected from the various scientific journals and 

 transactions through which they are dispersed, and digested in a didactic 

 order, seemed to be particularly wanting, and to be especially calculated 

 to further the objects of the Society for the Diffusion of Useful Knowledge. 

 In pursuance of this design, I have aimed at giving to the subjects treated as 

 much condensation as was compatible with perspicuity. I have endeavoured to 

 conduct the student, by a regular progression, from the simpler to the more 

 complex topics of research ; and I have also been anxious, by placing con- 

 stantly before his view the distinction which exists between ascertained facts, 

 and the hypotheses and theories devised for their explanation, to illustrate the 

 precepts of Bacon by examples, and to foster that genuine spirit of philosophi- 

 cal inquiry by which alone error can be avoided, and truth attained. 



For the many deficiencies which I fear the reader will discover in the 

 completion of this design, I have to plead, in extenuation, the very scanty 

 portion of leisure, which the continual pressure of my professional duties 

 leaves at my disposal. When I undertook this task, at the request of the 

 Society, above four years ago, I was far from anticipating the extent of the 

 labour it has imposed upon me; and from the multiplied interruptions to 

 which I have been subject, I have been compelled to prosecute the work in a 

 desultory manner, and at irregular and uncertain intervals. 



Since the publication of the earlier Treatises, many valuable researches 

 have been made, both in Electricity and in Galvanism, which deserve to be 

 recorded in their proper places. This, however, is an inconvenience which, 

 in the present age of improvement, must be incident to every scientific 

 Treatise ; for while so many accessions are daily accruing to the stock of 

 information, it is hardly possible to keep pace with the rapid growth of 

 knowledge ; nor can we ever hope to incorporate the whole of the disco- 

 veries, which have been made up to the last moment of publication, in a 

 systematic work on any science. To wait till perfection is attained would be 

 vain and fruitless presumption ; for the architecture of science has this pecu- 

 liarity, that the foundations must be prepared, and the superstructure begun, 

 long before the plans and elevations are completed. To posterity will be left 

 the task of adding the key-stone, and of removing the scaffolding which 

 interrupts the symmetry of the perfect edifice. 



P. M. ROGET. 



39, Bernard- Street, Russell-Square, 

 December 12^,1831, 



