Polytechnic Association Proceedings. 351 



under such conditions, one square foot of surface would be as effi- 

 cient, as is the two hundred square feet of an ordinary fifty-horse 

 boiler, and I believe that I shall be able to satisfy you that such, 

 or nearly such, results are attainable. 



But do not let your attention be disturbed by supposing that I 

 am about to propose maldng steam in small tubes, or upon flat 

 surfaces, or in any other of the thousand and one ways already 

 tried and found wanting, for as I told you at the first, I shall pro- 

 pose an entirely new principle as applied to the steam boiler. But 

 first, we will learn what a given metallic surface, under the most 

 favorable conditions, can accomplish, and having placed our mark 

 high, endeavor to come as near to it as the known laws of mechanics 

 will admit. 



You are aware that flame cannot pass through the meshes of 

 wire cloth (this is the principle of the safety lamp), not because it 

 ofiers resistance to the gas, but the metal absorbs the caloric so 

 rapidly that the temperature is reduced below the point of com- 

 bustion, before it has time to get through; and if these wires were 

 tubes, through which a stream of water was constantly passing, the 

 heat would be so much the more readily absorbed. 



We sec then that a surface can be so arranged as to absorb caloric 

 as fast as an equal area of combustion can generate, or at least very 

 nearly so. 



In some of our locomotives, a hundred and fifty pounds of coal 

 per hour are consumed on a square foot of grate. This is equal, 

 in actual practice, to the evaporation of more than a thousand 

 pounds of water, or fifty-horse power. Put these two facts together, 

 and a fifty-horse boiler should, when perfect, have one square foot 

 grate, one of evaporating surface, and a steam dome. We will 

 now compare a tubular boiler (the best form known at present to 

 the profession), and see what percentage of its evaporating sur- 

 face accomplishes this maximum amount of duty. 



The steam, formed at the bottom, is converted into mechanical 

 motion in the boiler, and lost, as I will illustrate. Take a vessel of 

 water several feet in depth, representing a section of our boiler, 

 and apply heat to the bottom until it boils fiercely, you will find 

 that ver}'- little, if any, evaporation takes place beyond that due to 

 atmospheric absorption. It takes some force to keep a body of 

 water in rapid motion, and more especially when it is under a 

 pressure of ninety pounds steam, equal to another column of water 

 two hundred feet high. Now, pour a little water into the bottom 



