THE STEAM-ENGINE. 129 



water surface. Another is that when the water is foul, the 

 tubes become so coated with deposit as to fill up the spaces 

 between them almost to a solid, and you cannot clean them. 

 But another and a very important reason is this through 

 these tubes have to go the products of combustion. Those 

 products commonly have not completed their combustion 

 in the fire-box, but you require it to be completed in the 

 tubes ; and if they are too small you have made your boiler 

 into the meshes of a Davy lamp, and by means of these 

 tubes you have as effectually snuffed out and extinguished 

 the gases which go through them, as the meshes of a Davy 

 lamp extinguish the inflammable gases which endeavour 

 to pass through them. You therefore have very good reasons, 

 all of them, why the multitubular system should not be 

 carried too far. In practice in steam-boats the tubes are 

 generally from 2J to 3J inches in diameter, with probably 

 from J to 1J inches of water space and with a length of tube 

 of 6 ft. 6 in. In locomotives, where you have a forced draft and 

 therefore do not require so large an area to get the products of 

 combustion through, the tubes average probably from 1J to 

 If inches diameter, and their length varies from 8 to 10 feet. 

 The tubes of the portable agricultural engine lie between 

 these dimensions. To-morrow I hope to say a word to you 

 about the proportion of surface, but in speaking of the actual 

 proportion I am inclined to state that, as far as my experience 

 goes, it matters very little indeed, except upon the scores of 

 safety and ability to clean, and of having a large water 

 surface to deliver steam, what kind of boiler you employ, 

 so long as it has an adequate absorbing surface for the heat 

 evolved from the fuel in a given time. I believe the various 

 contrivances or notions about vertical surfaces and flat surfaces, 

 and all these things which have agitated engineers' minds for 

 many years, are absolutely unimportant so long as you have 

 an adequate absorbing surface for the heat produced. It is much 

 more important to have a construction which will afford dry 

 steam, power of cleaning, and of repair. I will now ask you to 

 consider a little how much of a successful economic result is 

 due to the proportioning of the boilers, and to good stoking. 

 On the wall is a table, which is only a small portion of one 

 published by the Eoyal Agricultural Society relating to the 

 performance of portable engines under trial by that 'Society. 

 You will find two columns showing the number of pounds of 



