292 
than can be readily conceded, they will not be sufficient to 
account for so Jarge an economy as is said to be realised. 
Secondly, it is argued that the bubbles of air virtually afford an 
extension of heating surface. So they do, in relationto the heat 
carried in by the air ; but the air can only part with its heat by 
lessening its direct contribution to the power of the engine. 
Moreover, if the heat carried in by the air be insignificant in 
quantity, as I believe it to be, the explanation fails in every poiat 
of view. 
Thirdly, it is stated that the action of the air prevents and even 
removes incrustation, and thereby keeps the heating surfaces free 
from all obstruction as regards the transmission of heat. Very 
careful observation would be required to establish this fact ; but, 
granting the fact, it would follow that the advantage of injecting air 
would be limited by those cases in which deposit would otherwise 
be formed. Ina boiler perfectly free from incrustation the injection 
ofair ought to be nugatory, but this does not appear to be the case. 
Fourthly, it has been ingeniously suggested by Mr. Siemens 
that the air passing with the steam into the cylinder may form a 
film on the interior surface capable of arresting, in a great mea- 
sure, that condensation which is known to be so wasteful of 
power in unjacketed cylinders, where the steam is used expan- 
sively. It is highly probable that the air would really accumu- 
late in this manner against the sides of the cylinder ; because, 
while the particles of steam sank down into water, the particles of 
air wouldremain. It is also pretty clear that this film of air would 
intercept the abstraction of heat by the cooled material of the 
cylinder ; but if we admit this mode of action, then it would seem 
to follow that it is only in the absence of a steam jacket to the 
cylinder that the economy of injecting air is realised, and in fact 
that the injection of air is merely a substitute for steam jacketing. 
Moreover, if such be the action of the air, pumping into the 
steam should, in this point of view, produce the same effect as 
pumping into the water. 
I have dilated upon this subject more, perhaps, than necessary, 
but I have done so with a view to stimulate action in the matter, 
for it is time that the doubts and obscurities which beset the 
system should be cleared up, and its adoption or rejection be 
brought to an issue. There is no class of steam-engine in which 
economy of fuel is of so much importance as it is in marine 
engines, for not only is it an object in steam navigation to di- 
minish the cost of coal, but it is a still greater object to save 
room, and thereby increase the space available for cargo. The 
introduction of compound engines has enabled steam to be used 
of much higher pressure than formerly, and with greatly in- 
creased expansive action. The result has been a saving of about 
50 per cent. in the consumption of coal, and I believe I am sub- 
stantially correct in saying that in steam vessels, employed on 
long voyages, this saving of coal has been attended with a four- 
fold increase of the previous carrying power. It is highly 
probable that still further reductions of fuel will be effected by 
following in the same path, which has already lead to such great 
economy. ‘The pressure of steam in marine engines is still far 
inferior to that which is used in locomotive engines, and there is 
no obstacle, of an insurmountable nature, against the expansive 
action being increased proportionately to any further increase of 
pressure. 
But our efforts to increase the efficiency of marine engines 
must not run too much in one groove. Recent improvements 
have been almost exclusively directed to the mode of aff/ying 
the steam, and but little attention has been paid to the mode of 
producing it. The engine has advanced enormously in improve- 
ment, but the boiler has actually receded ; for we now get less 
evaporative effect from marine boilers than was obtained from 
those previously in use. This diminution of effect has resulted 
from changes made in the form of the boiler, to enable it to resist 
the greater pressure of the steam ; but there is no inherent ne- 
cessity for sacrificing evaporative power to meet this requirement, 
as is proved by the example of the locomotive boiler, which, 
while it produces steam of double the pressure of that supplied 
by marine boilers, stands unrivalled in regard to evaporative 
effect. The superiority of the locomotive boiler in regard to 
evaporating power is chiefly due to the large capacity of its fire- 
box, which affords ample space above thie surface of the fuel for 
perfecting the combustion of the gases. In the old form of 
marine boiler the flame space above and beyond the fire was also 
very large, and the evaporation per pound of coal was nearly as 
great as in the locomotive. But this advantage has been sacri- 
ficed in the modern form of boiler, by adopting a cylindrical 
fire chamber within the boiler, This form is very favourable to 
NATURE 
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[Fes. 13, 1873 
strength, but it affords very little head-room over the fire, and 
the consequence is that, although the tubular heating surface is 
relatively as great as before, the evaporation per pound of coal 
has fallen considerably. I do not say that the locomotive form 
of boiler, pure and simple, is that which ought to be adopted 
for marine engines, but it is well worth consideration, whether 
by adopting the same principle of gonstruction, a more efficient 
boiler would not be obtained fer marine engines. A more 
powerful draught would probably be required than is now neces- 
sary, but this could be obtained by known mechanical methods, 
applied either to draw air through the furnaces, or to force it 
into a closed stoke-hole. The production of draught by aux- 
iliary power, would have the great advantage of enabling the 
rate of combustion to be regulated at pleasure, so as to meet the 
varying demand for steam, and it would also facilitate the appli- 
cation to marine boilers of mechanical firing, which does not 
succeed with a slow draught, and requires a variable draught to 
meet the fluctuating production of steam required at sea. The 
great number of stokers required in large steamers, the severity 
of the work, and the inefficiency of the method they pursue, as 
evidenced by the dense clouds of smoke they produce, render 
the introduction of mechanical firing in such vessels a matter of 
the utmost importance; and I do not believe that any of the 
difficulties which appear to stand in the way are incapable of 
removal. 
I must not dismiss the subject of steam power without some 
allusion to its application to agriculture. In no description of — 
steam-engine has economy of fuel been more perseveringly and 
successfully followed out than in engines for agricultural use ; 
and Mr. Bramwell, in his late address to the Mechanical Section 
of the British Association, does full justice to the mechanical 
engineers who have ben the means of bringing these engines to 
such a high degree of efficiency. It is satisfactory to see that 
the application of steam to the cultivation of the land, and to 
every kind of farming operation, is rapidly extending ; for if the 
food producing power of the land has to be increased, it must be 
by substituting, as far as possible, the comparatively cheap 
power of steam, for the labour, both of men and horses, The 
greatly increased demand for labour in manufacturing occupa- 
tions, as well as for mining and constructive purposes, will cer- 
tainly diminish the supply of rural labour and increase its cost. 
Such a result is not to be regretted, considering how miserably 
ill requited farm labour in most parts of England has been ; but 
unless the growing cost of agricultural labour and of horse work 
can be counterpoised bya more extensive use of steam power, 
we may expect much of the land in this country to be thrown 
out of cultivation. Very different are the views of those who 
maintain that food would be more economically produced by 
increasing, instead of diminishing, the labour employed on the 
land. Such is the doctrine of those who advocate the parcelling 
out of the land in small plots to peasant holders, and who even 
contend that waste lands, incapable of profitable return by ordi- 
nary treatment, could, by this means, be advantageously culti- 
vated. It would, indeed, be a retrograde step to renounce the 
aid of capital and mechanical skill in tillage, and fall back upon 
the primitive system of spade husbandry. If there be a country 
in the world where such a mode of cultivation is the best, that 
country is assuredly not England, where all the resources of 
science and skill are necessary to the maintenance of a large 
population, under adverse conditions of soil and climate, and 
where labour is more highly paid in manufacture than in agricul- 
ture. 
I have had considerable personal experience of steam cultiva- 
tion, and am a thorough believer in its efficacy ; but I may here 
draw attention to a very general subject of complaint concerning 
the machinery and implements employed for the purpose. 
refer to the frequency of breakages due to insufficient strength in 
the construction. If makers of the apparatus, used in all the 
varieties of steam tillage, could only be induced to be more 
liberal in the use of material, the introduction of their machines 
would be very greatly accelerated. 
I must also touch upon the subject of steam traction on com- 
mon roads, which has lately received a considerable impulse from 
the introduction of Mr. Thomson’s invention of India-rubber tyres. 
The number of horses in this country is enormous, and being great 
consumers of food, their maintenance isa heavy charge on the re- 
sources of the nation, Next to human power, horse power is the 
most expensive that we can use, and we may welcome the dawn 
of a period when steam will, to a great extent, supplant animal 
power in our streets and highways. 
