on the Steam Engine. 325 



Upon the subject of employing tubes instead of boilers, upon 

 which so much was said a few years since, our author offers the fol- 

 lowing remarks. 



As the quantity of steam generated, depends wholly upon the sur- 

 face of the boiler that is exposed to heat, and as the saving of weight 

 is, in many cases, advantageous, it has been proposed to use a combi- 

 nation of tubes for boilers, which will expose a much greater surface, 

 hi comparison with theirlnternai capacity, than larger cylinders ; for 

 it is a mathematical law, that while the surfaces of cylinders of equal 

 length increase as the diameters simply, their internal capacity in- 

 creases with the squares of that dimension. A saving may also be 

 made in the material of which the tubes are constructed, for the 

 strength of a metallic tube to resist an effort tc» burst it, increases in 

 the inverse ratio of its diameter. It has also been proposed to im- 

 merse such tubes wholly in the flame, and inject into them, from time 

 to time, a certain quantity of water, to be converted almost instantly 

 and wholly into steam. Such were the original boilers of Babcock. 

 The first of these plans has a speedy limit in practice, and the last 

 is wholly inadmissible, as will appearfrom the following considerationsi 



1. The presence of a conducting body in the midst of the flame, 

 will cool the gas of which it is composed, diminish the intensity of 

 the combustion, and the draught of the chimney. 



2. When tubes are actually heated to the proper degree, and no 

 longer act to cool the flame, the flues must be made short enough to 

 permit the air to enter the chimney as soon as it is cooled down 

 to the temperature of the tubes, otherwise, instead of heating them 

 farther, it will tend to cool them. — pp. 72 and 73» 



Another very serious objection to the use of tubes in the place of 

 boilers (the tubes being immersed in the flame,) is found in certain 

 anomalous effects produced upon steam when brought into contact 

 with a highly heated surface, effects to which we shall advert more 

 particularly by and by. It is an additional objection to tubes, that 

 •the deposits of solid matter, which fall from almost all water when 

 evaporated, and which are greater in proportion as the water is more 

 impure, become harder and more compact than when the boiler is 

 kept full of water. They also adhere more forcibly to the metal, 

 and are more hable to corrode it. The author, however, concedes that 

 this method has the advantage of being free from all risk of explo- 

 sion, and that there are of course cases where this advantage may 

 be worth obtaining, even at the sacrifice of a considerable quantity 

 of heat. 



Vol XX.™-No. 2, 42 ■ 



