XXVlll INTRODUCTION. 



engines, apparatus, and instruments employed, not only in the 

 higher departments of natural philosophy, but in the every-day 

 concerns of society, in the arts, manufactures, and domestic 

 operations of civilized men. The occurrence of such principles 

 seems to present the legitimate time and place for classifying the 

 inventions to which they gave existence, and for directing genius 

 in its attempts to elicit new applications of collateral principles : 

 for though fortuitous circumstances and accidental hints may 

 have led to some discoveries in Hydrodynamics, the greater part 

 of modern improvements must be traced to patient induction, 

 which arrives at those coincidences whereby scientific men are 

 enabled to expound the theory of particular machines, whose 

 construction and principles of action depend upon the equi- 

 librium or motion of fluids. By this method, nothing is taken 

 for granted which can be investigated from a series of mathe- 

 matical truths ; for, as Mr. Whitehurst observes, " it is one thing 

 to assent to truths, and another to prove them to be true : the 

 former leaves the mind in a state of suspense, the latter in the 

 possession of truth." * 



This chapter concludes with some experiments upon the 

 quaqua versus property of non-elastic fluids ; these experiments 

 have the lowly merit of placing that property, the power of 

 transmitting pressure equally in all directions, in a popular 

 point of view, " level to the capacity of ordinary minds." 



Our labours hitherto refer exclusively to what may be termed 

 elementary principles in the mechanics of fluids ; we now com- 

 mence with PRESSURE, as it unfolds itself in the action of fluids 

 of variable density,^ or such as have their densities regulated by 

 certain conditions, dependent upon particular laws, whether ex- 

 cited by motion, by mixture, or by change of temperature. This 

 is the subject of Chapter Seventh, in which it will be found 

 that the investigation of the pressure of fluids of variable den- 

 sity is fruitful of some remarkably curious results : among these 

 we may notice the circumstance of a globe of condensible 



* " Inquiry into the Original State and Formation of the Earth." London, 1792. 



t The word variable is perhaps taken in a too general sense : the densities are not 

 variable in all cases, they are only different yet they are sometimes variable also ; 

 but there can be no more correct mode of writing upon this subject. 



