CHAPTER VII. 



OF 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 PAR- 

 TICULAR LAWS, WHETHER EXCITED BY MOTION, BY MIXTURE, 

 OR BY CHANGE OF TEMPERATURE. 



IN the former part of this treatise, we have displayed the nature of 

 pressure as it occurs in the action of non-elastic fluids of uniform 

 density, and in addition, we have investigated the theory and exem- 

 plified the application of the Hydrostatic Press, the Hydrostatic 

 Bellows, and the Hydrostatic Balance or weighing machine ; instru- 

 ments whose operations depend upon the quaqua-versus principle of 

 non-elastic and incompressible fluids: We come therefore in the 

 next place, to consider 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 excited 

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



In mechanical science density is used as a term of comparison, 

 expressing the proportion of the number of equal moleculee in the 

 same bulk of another body; density, therefore, is directly as the 

 quantity of matter ; and inversely as the magnitude of the body* 



We cannot by means of our senses discover the figure and magnitude of the ele* 

 mentary particles of matter. Mechanical inventions have wonderfully magnified 

 objects invisible to the unassisted eye; but no microscopical assistance has yet en- 

 abled us to assume that we have seen an elementary particle of matter. A number of 

 elementary particles uniting bv the power of cohesion form greater particles, and 

 these again uniting, by the same power, form still greater; and we may consider the 

 aggregate of many such formations to become at length an atom of a sensible bulk. 

 All bodies seem to be composed of these derivative corpuscles, which, formed of 

 more or fewer of these repeated unions, compose bodies more or less dense. These 

 derivative corpuscles are sometimes similar, as the coloured rays of a beam of light, 



VOL. I. M 



