AUTHOR'S PREFACE. ix 



parhelia have been explained upon principles not entirely new, but long 

 forgotten ; the functions of the eye have been minutely examined, and the | 

 mode of its accommodation to the perception of objects at different dis- \ 

 tances ascertained ; the various phenomena of coloured light have been 

 copiously described, and accurately represented by coloured plates ; and 

 some new cases of the production of colours have been pointed out, and 

 have been referred to the general law of double lights, by which a great 

 variety of the experiments of former opticians have also been explained ; 

 and this law has been applied to the establishment of a theory of the nature 

 of light which satisfactorily removes almost every difficulty that has 

 hitherto attended the subject. 



The theory of the tides has been reduced into an extremely simple 

 form, which appears to agree better with all the phenomena than the more 

 intricate calculations which they have commonly been supposed to require. 

 With respect to the cohesion and capillary action of liquids, I have had 

 the good fortune to anticipate Mr. Laplace in his late researches, and I 

 have endeavoured to show that my assumptions are more universally 

 applicable to the facts, than those which that justly celebrated mathema- I 

 tician has employed. I have also attempted to throw some new light on 

 the general properties of matter in other forms ; and on the doctrine of 

 heat which is materially concerned in them ; and to deduce some useful 

 conclusions from a comparison of various experiments on the elasticity of 

 steam, on evaporation, and on the indications of hygrometers. I have 

 enumerated, in a compendious and systematical form, the principal facts 

 which have been discovered with respect to galvanic electricity; and I 

 have fortunately been able to profit by Mr. Davy's most important expe- 

 riments, which have lately been communicated to the Royal Society, and 

 which have already given to this branch of science, a much greater per- 

 fection, and a far greater extent, than it before possessed. The historical 

 part of the work can scarcely be called new, but several of the circum- 

 stances which are related, have escaped the notice of former writers on the 

 history of the sciences. 



Besides these improvements, if I may be allowed to give them that 

 name, there are others, perhaps of less importance, which may still be 

 interesting to those who are particularly engaged in those departments of 

 science, or of mechanical practice, to which they relate. Among these 

 may be ranked, in the division of mechanics, properly so called, a simple 

 demonstration of the law of the force by which a body revolves in an 

 ellipsis ; another of the properties of cycloidal pendulums ; an examina- 

 tion of the mechanism of animal motions ; a comparison of the measures 

 and weights of different countries ; and a convenient estimate of the effect 

 of human labour : with respect to architecture, a simple method of 

 drawing the outline of a column : an investigation of the best forms for 

 arches ; a determination of the curve which affords the greatest space for 

 turning ; considerations on the structure of the joints employed in car- 

 pentry, and on the firmness of wedges ; and an easy mode of forming a 

 kirb roof : for the purposes of machinery of different kinds, an arrange- 

 ment of bars for obtaining rectilinear motion ; an inquiry into the most 



