10 LECTURE I. 



the affections of liquids at rest ; pneumatostatics, or the properties of elastic 

 fluids at rest ; and hydraulics, or the theory of fluids in motion. The 

 practical application of this theory to hydraulic and pneumatic machines 

 is of very considerable importance, and is as interesting to the philosopher 

 as it is necessary to the engineer. The employment of the force of water 

 and wind to the best advantage, the draining of lands and mines, the supply 

 of water for domestic convenience, the manoeuvres of seamanship, the con- 

 struction of the steam engine, are all dependent upon hydrodynamical 

 principles, and are often considered as comprehended in the science of 

 hydraulics. Harmonics and optics, the remaining parts of this division, are 

 more insulated : the doctrine of sound, the theory of music, and the con- 

 struction of musical instruments, are as pleasing to the intellect in theory, 

 as they are gratifying to the senses in practice ; but the science of optics is 

 not less interesting, and at the same time far more useful ; the instruments 

 which it furnishes are of almost indispensable necessity to the navigator, to 

 the naturalist, to the physiologist, and even to the man of business or plea- 

 sure. It is perhaps in this science that many persons of the greatest 

 genius have been the most happily employed. The reasons for which it is 

 classed as a division of hydrodynamics will be explained hereafter. 



The contemplation of the particular phenomena of nature, as they are 

 displayed in the universe at large, contributes perhaps less to the perfection 

 of any of the arts which are immediately subservient to profit or conve- 

 nience, than the study of mechanics and hydrodynamics. But the dignity 

 and magnificence of some of these phenomena, and the beauty and variety 

 of others, render them highly interesting to the philosophical mind ; at the 

 same time that some of them are of the utmost importance in their appli- 

 cation to the purposes of life. In all these respects the science of astro- 

 nomy holds the first rank ; its uses in assisting navigation, and in regulating 

 chronology, are beyond all calculation. Geography and hydrography, or 

 the particular histories of the earth and sea, are immediately connected 

 with astronomy. The discussion of the properties of matter in general, 

 and of the alterations of temperature to which all bodies are liable, has not 

 hitherto received a distinct appellation as a science ; but both these subjects 

 require a separate consideration, and afford a vast scope for speculation 

 and for observation. Electricity and magnetism are partly referable to the 

 affections of matter, and partly to the agency of substances which appear 

 to agree with common matter in some properties and to differ from it in 

 others. The phenomena produced by these agents are often such as excite 

 a high degree of curiosity to inquire into their causes, although the inquiry 

 too often terminates only in astonishment ; but we have reason to expect 

 considerable advancement in these sciences from the singular discoveries of 

 modern chemists. The utility of the philosophy of electricity is sufficiently 

 exemplified in the general introduction of conductors for securing us 

 against lightning, to say nothing of the occasional enployment of electricity 

 in medicine ; and since the important discovery of the compass, we have 

 only to lament that the changeable nature of magnetic effects so much 

 limits the utility of that instrument for nautical and geographical purposes. 

 Of meteorology and of geology our knowledge is hitherto very imperfect. 



