356 On the Expenditure of Heat in the Hot-air Engine. 
vantage is somewhat greater than it appears in the table, since 
this column is computed by regarding the expansion of steam as 
conforming to the law of Mariotte. 
There are large drawbacks, however, which have not been con- 
sidered. In the first place, the regenerators cannot possibly (as 
our supposition has presumed) abstract from the escaping air all 
the heat down to the temperature ”. There must also be great 
loss by radiation and conduction. Much heat is carried off by 
the leakage of the heated air, and finally the escape of heat 
through the smoke pipes cannot but be largely greater than occurs 
in heating water in boilers. 
apt. Ericsson allows a loss of 30° from the first of the causes 
above named. This would amount to from to 25 per cent. 
upon the total theoretic consumption. In working with / =m 
it would be 20 per cent. ; and this alone would make a consider- 
able reduction upon the favorable ratios in the table. 
It will be seen, by inspection, that if power is sought without 
regard to economy, large supply cylinders are to be preferred. 
The construction proposed by the present writer in the last num- 
ber of this Journal, renders the use of such cylinders practicable, 
though it would not be so on Ericsson’s plan. 
If economy is sought at the expense of power, cylinders from 
‘60 to 2 in cross section, furnish the best results. In this point 
of view, Capt. Ericsson’s proportions are well chosen. It is also 
apparent that the cut-off which produces the most economical re- 
sults is at ‘75 or thereabouts, whatever be the ratio between the 
cylinders. 
But, on all accounts, it is evident at a glance, that the supply 
cylinders between ‘70 and ‘80 in section (probably about ‘75 in 
preference to any other) are most eligible. Very little is lost by 
their use, in point of economy, while there is secured a very great 
gain of power. 
But here probably we encounter the great and it is to be ap- 
prehended the inseparable obstacle, in the way of the success of 
the hot-air engine viz., the practical difficulty of heating the 
great mass of air with sufficient rapidity. Suppose we make 
all reasonable allowances for the effect of friction, leakage, and 
clearance of cylinder, it is not possible after all to bring down the 
power so low as the experiment with the “ Ericsson” ship showed 
it in fact to be. 
Capt. Ericsson, for instance, says that a pressure of half a pound 
to the inch, is sufficient to overcome resistances. At 12 pounds 
above the atmosphere in the reservoir (what he aimed at), this 
would constitute a charge of about 15 per cent. upon the power ; 
for with 12 pounds actual pressure, but about 3:4 Ibs. pressure 
over resistances is obtained. For leakage we have no reliable 
a upon which to form an estimate; but we know that in the 
