as a Vehicle for transporting Heat. 331 



The strength of bodies of different sizes, similar in form 

 and composed of the same substance^ or the forces by 

 which they resist being pulled asunder by weight sus- 

 pended to them, and acting in the direction of their 

 lengths, is not in the simple ratio of the areas of their 

 transverse sections, or of their fractures^ but in a higher 

 ratio ; and this ratio is different in different sub- 

 stances. 



The form of a body has a considerable influence on 

 its strength, even when it is -pulled in the direction of its 

 length. 



All bodies, even the most brittle, appear to be torn 

 asunder^ or their particles separated, or fibres broken, 

 one after the other ; and hence it is evident that that form 

 must be most favourable to the strength of any given 

 body, pulled in the direction of its length, which enables 

 the greatest number of its particles, or longitudinal 

 fibres, to be separated to the greatest possible distance 

 short of that at which the force of cohesion is over- 

 come, before any of them have been forced beyond that 

 limit. 



It is more than probable that the apparent strength 

 of different substances depends much more on the 

 number of their particles that come into action before 

 any of them are forced beyond the limits of the attrac- 

 tion of cohesion, than on any specific difference in the 

 intensity of that force in those substances. 



But to return to the subject more immediately under 

 consideration. As it is essential that the steam em- 

 ployed in heating liquids, in the manner before described, 

 should enter the containing vessel at or very near its 

 bottom, it is evident that this steam must be sufficiently 

 strong or elastic to overcome not only the pressure 



