of tons annually, it is demonstrable that the coal-fields of England and the 

 United States would not be exhausted for many centuries. 



But in speculations like these, the probable if not certain progress of im- 

 provement and discovery ought not to be overlooked ; and we may safely pro- 

 nounce that, long before such a period of time shall have rolled away, other 

 and more powerful mechanical agents will supersede the use of coal. Phi- 

 losophy already directs her finger at sources of inexhaustible power in the 

 phenomena of electricity and magnetism. The alternate decomposition and 

 recomposition of water, by magnetism and electricity, has too close an analogy 

 to the alternate processes of vaporization and condensation, not to occur at 

 once to every mind : the development of the gases from solid matter by the 

 operation of the chemical affinities, and their subsequent condensation into the 

 liquid form, has already been essayed as a source of power. In a word, the 

 general state of physical science at the present moment, the vigor, activity, 

 and sagacity, with which researches in it are prosecuted in every civilized coun- 

 try, the increasing consideration in which scientific men are held, and the per- 

 sonal honors and rewards which begin to be conferred upon them, all justify 

 the expectation that we are on the eve of mechanicaf discoveries still greater 

 than any which have yet appeared ; and that the steam-engine itself, with the 

 gigantic powers conferred upon it, will dwindle into insignificance in compari- 

 son with the energies of nature which are still to be revealed ; and that the 

 day will come when that machine, which is now extending the blessings of 

 civilization to the most remote skirts of the globe, will cease to have existence 

 except in the page of history. 



In explaining the different forms of steam-engine which have been proposed 

 in the course of the progressive improvement of that machine from its early 

 rude and imperfect state to its present comparatively perfect form, it will be 

 necessary to advert to physical phenomena and mechanical principles, which, 

 however obvious to those who are conversant with matters of science, must 

 necessarily be at least imperfectly known by the great majority. To refer for 

 information on such topics to other works on mechanics and general physics, 

 would be with most readers ineffectual, and with all unsatisfactory. We shall 

 therefore pause as we proceed, where these difficulties occur, to give such ex- 

 planation and illustration as may seem best suited to render them intelligible 

 and interesting to the unscientific reader. 



Fluid bodies are of two kinds inela'stic fluids, or liquids, and elastic fluids, 

 or gases. Of the former of these classes, water is the most familiar example ; 

 and of the latter, air. 



These two species of fluids are each distinguished by peculiar mechanical 

 properties. 



The constituent particles of a liquid are distinguished from those of solids 

 by having little or no coherence ; so that unless the mass be confined by the 

 sides of the vessel which contains it, the particles will fall asunder by their 

 gravity. A mass of liquid, therefore, unlike a solid, can never retain any par- 

 ticular form, but will accommodate itself to the form of the vessel in which it 

 is placed. It will press against the bottom of the vessel which contains it 

 with the whole force of its weight, and it will press against the sides with a 

 force proportional to the depth of the particles in contact with the sides meas- 

 ured from the surface of the liquid above. This lateral pressure also distin- 

 guishes liquids from solids. Let us take for illustration the case of a square 

 or a cubical vessel, A B C D, fig. 1. If a solid body, such as a piece of lead, 

 be cut to the shape of this vessel, so as to fit in it without pressing with any 

 ,' force against its sides, the mechanical effect which would be produced by it 

 < when placed in the vessel, would be merely a pressure upon the bottom B C, 



