SECT, iv.] AND PROPORTIONS OF ENGINES. 161 



necessarily successive to render it rectilineal ; but all the species of action are used 

 either jointly or separately. From these species of action, therefore, the engines may 

 be classed. 



322. It will be remarked that steam must be either condensed, or generated 

 under pressure, to afford power by expansion ; hence engines may be divided 

 into two classes, depending on condensation being used, or not ; this arrangement 

 being most convenient. 



CLASSIFICATION OF STEAM ENGINES. 



323.-I. Non-condensing engines acting by the / l ' g enerative P ower of steam. 



12. generation and expansive power of steam. 



C\. condensation of steam. 



324. II. Condensing engines acting I 2. condensation and expansion of steam, 



by the - j 3. generation and condensation of steam. 



[_4. generation, expansion, and condensation of steam. 



325. All the engines of the first class, and the third and fourth kinds of the 

 second class, require high pressure steam. Engines of the first class are remarkable 

 for simplicity of construction, but they never give the whole power of the steam. 

 Engines of the second class require a considerable quantity of cold water for con- 

 densation, and therefore in some cases cannot be applied. The greatest effect is 

 obtained by the second and fourth kinds of the second class ; or rather it is only 

 in these two species that the whole power of the steam is obtained. 



326. In both classes there are certain proportions between the length of the 

 stroke, and the diameter of the cylinder, and between the length of the stroke 

 and the velocity, which give a maximum of useful effect to a given quantity of 

 steam. These being considered, and also the proportions of the additional parts 

 required in condensing engines, the general rules for the power of engines may 

 be established. 



i 



OF THE RATIO BETWEEN THE LENGTH OF THE STROKE, AND THE DIAMETER 



OF THE CYLINDER. 



327. The relation between the diameter of a steam cylinder and the length 

 of the stroke, and consequently the proportions of the cylinder, have now to be 

 considered. If all the apertures, and all other parts be duly proportioned, and 

 the velocity regulated so as to be esteemed uniform, then there is no circumstance 

 relating to the motion, which has any influence on the proportions of the steam 



x 



