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. ' genet 
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, 
in comparison with their internal capacity, than larger cylinders ; for 
itis 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 to 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 considerations. 
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 
j _ boilers (the tubes being immersed in the flame,) is found in certain 
atomalous 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 
deposits of solid matter, which fall from almost all water when 
*Yaporated, and which are greater in proportion as the water is more 
. pure, become harder and more compact than when the boiler is 
Kept full of water. They also adhere more forcibly to the metal, 
and are mdre liable 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 
es obtaining, even at the sacrifice of a considerable quantity 
Vol. XX.—No. 2. 42 
