Prof. Barnard on the Elastic Force of Heated Air. 165 
t = 1085°°07 = 626°°07 F. 
m 1 P Pp ] R R’ 
Joule, ‘750 -333..7:33 70-56. 36498 2877 4-212 
Max. m, 917 +333 787 8626 403 -7 3-087 4-530 
Max. J, 750 -420 763 55.98 301 8. 2-507 3-670 
Abs. max, 1021 -478 876 67:00 342 6, 2-412 3-539 
t == 941 = 482° F. , 
Joule, ‘750 ‘A472 461 43:22 246°-7 2-086 3-054 
Max. m, ‘928-472 «5:14 63-49 291 9 2325 Bagg 
Max. J, ‘750 -488 465 42:00 241 0 2-054 3-007 
Abs. max, *960 “538 516 48°55 271 -5 2-034 ° 2-977 
It appears from these summaries, that the arrangement most fa- 
vorable to economy is that in which, the cut-off remaining where 
Mr. Joule’s principle would fix it for the given supply cylinder, 
thissame supply cylinder is enlarged to the dimensions which 
give the maximum of power for that cut-off. It also appears that 
the conditions which furnish the absoluée maximum of power for 
the temperature, are the least economical of those which the ta- 
ble embraces. . 
Two double action cylinders fixed at right angles to each oth- 
er, with their piston rods attached to the same crank, will, with 
the arrangements proposed in my September article, preserve a 
preponderance of positive power throughout the stroke, which- 
ever of the above proportions are adopted under the temperature 
of 482° F, 
For the higher temperatures, three cylinders, or four, would 
be necessary ; unless some new mechanical expedient should. be 
resorted to, to reduce the resistance at the énd of the stroke. 
Such an expedient it was a part of the design of the present article 
to propose—not as the best possible, for it has not been a sub- 
ject of much study—but merely for the sake of illustrating the 
practicability of overcoming what has been regarded as a formida- 
le difficulty in the way of the success of the air engine. It 
would extend this article, however, beyond reasonable limits, to 
attempt a particular description of the contrivance here. It ma 
be sufficient to say that it is a contrivance for causing the piston 
at the period of maximum power to encounter and overcome the 
Maximum resistance :—that is to say, to throw the burthen of 
driving into the reservoir a charge of air fully condensed by a 
previous stroke, upon the first third of each stroke, (or first fourth, 
if Mr. Jonle’s proposed temperature be adopted,) and to leave to 
the last two-thirds, only the labor of bringing a new charge to the 
required density.* By this means, the great resistance which, 
*It is obvious that this might be effected, by so adinsiing the cranks that the 
driving piston ma: begin its stroke just as the air in the sappy ayia reaches the 
maximum*of condensation. It is desirable, however, to avoid the severe strain 
on the shaft which this plan would h ge the method hinted at in the 
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