CHAP. VIII.] 



STEAM NAVIGATION. 



455 



at mean and high pressure. The abundance of water renders the 

 employment of condensers easy and economical. 



The steam-engines employed in navigation are the most powerful 

 constructed. It is not rare for their effective force to be measured 

 by hundreds of horse-power. In some ships of the navy horse- 

 power is indeed counted by thousands. We must add that the 

 estimation of the power of the engines in horse-power what is 

 called their nominal force is made differently for these and for land- 

 engines. The low-pressure horse-power, or nominal horse-power, in 

 shipping, means not 33,000 only, but more than 44,000 foot-pounds, 



FIG. 309. Tubular boiler, with return flame, of 

 the I sly : section. 



FIG. 310. Marine tubular boiler with return 

 flame : section. 



the mean being 47,000 on the shaft and 59,400 on the pistons. The 

 reason of this is that the loss of motive power in the recoil has forced 

 constructors to exaggerate the force in view of the useful effect to be 

 produced. Even the numbers we have just given are now too small ; 

 in the United States shipping the horse-power nominal reaches 

 182,000 foot pounds. 



At this rate the steam frigate Friedland, whose engines have an 



