NATURAL PHILOSOPHY. 163 



in the water which has accomplished the condensation, exhibit, when the 

 power was turned into heat, a larger amount of heat than the steam contains, 

 or the engine would be a heat-generating engine, which is impossible. The 

 amount of power produced in a steam engine, therefore, is measurable by the 

 amount of heat wliich has disappeared from the hot well, or, in other words, 

 which cannot be discovered in the water by which the condensation of the 

 steam has been accomplished ; and in a perfect engine, hi which the whole 

 heat was turned into power, there would be no rise in the temperature 

 of the hot well at all over the temperature of the water admitted to perform 

 the condensation. The greater the difference of temperature between the 

 boiler and condenser, the more effectual will any given quantity of coals 

 be in generating power ; and it is because air admits of the use of a far higher 

 temperature than is possible hi the case of steam, that it realizes a very 

 superior economy. There are constructive impediments to the employment 

 of air engines which, however, are not very difficult of supersession ; and they 

 will be surmounted speedily, so soon as practical engineers are thoroughly 

 satisfied of the superior performance attainable by air engines, and which, 

 therefore, it is important widely to announce. Mechanical power being con- 

 vertible into heat, electricity, and also into light, it becomes easy to estimate 

 the mechanical value of those agents ; and a key is thus afforded whereby 

 these heretofore inscrutable departments of science may be brought under the 

 dominion of mechanical laws. Professor Kankine ascribes the elasticity of 

 gases to a centrifugal action of their particles ; and Professor Thomson, by a 

 very ingenious process, makes an estimate of the density of the ether, or 

 atmosphere, filling the interstellar spaces, by determining first the mechanical 

 value of a cubic mile of sunlight and the velocity of the vibrations by which 

 light is caused, and he knows then, by the usual laws of mechanics, that with 

 the given velocity of motion, the density must be such as to produce the 

 specified amount of mechanical power. 



ON THE PEACTICABILITY OF CONSTRUCTING CANNON OF GREAT 

 CALIBRE CAPABLE OF ENDURING LONG CONTINUED USE UNDER 

 FULL CHARGES. 



The following communication on the above subject was presented to the 

 American Academy, by Professor Treadwell of Cambridge : 



The importance of constructing cannon of a size larger than any now hi use, 

 to every nation that may be called upon to encounter the trials of war, is one 

 of those facts acknowledged alike by the soldier and the civilian ; and to 

 obtain such instruments, capable of throwing projectiles larger and heavier, and 

 to greater distances, than has hitherto been attained, is now occupying the 

 attention of the scientific engineers and projectors of Europe more than any 

 other question open to them. The present age has witnessed a remarkable 

 increase in the size of ah 1 the great instruments of human industry. Ships 

 within twenty years have been doubled in their dimensions, and steam engines 

 are now constructed which compare with those of the last age as giants compare 

 with common men. But although the want is fully acknowledged, and 



