LOW TEMPERATURES AND REFRIGERATION—MARCHIS. 209 
is to say, by a correlative destruction of the liquid air produced. 
Moreover, under the most favorable conditions the yield of this ma- 
chine is hardly more than 0.2 liter of liquid air per horsepower-hour. 
For this reason M. Claude has been led to modify the process in 
the following way, which I shall endeavor to explain: A part of the 
cooled current of air compressed to 40 atmospheres is deflected before 
it arrives at the expansion cylinder. This air under a pressure of 
40 atmospheres is led into a chamber (liquefier) cooled by the gas 
of the original current which has been expanded in the auxiliary 
motor. Thanks to a pressure of 40 atmospheres the deflected gas is 
liquefied in this latter chamber at a temperature no longer of —190°, 
as was the case in the original process, where liquefaction occurred 
at the limit of its expansion, but at —140°. Furthermore, the 
expanded air which circulated in the liquefier is heated up and arrives 
in the economizer no longer at —190°, but at about —130°. It cools 
the gas in the feed conduit less, so that this gas arrives in the auxil- 
lary motor at a temperature of about —100°. Its expansion takes 
place, therefore, under more favorable conditions, and liquefaction 
by expansion is less to be feared. 
By thus substituting liquefaction under pressure for spontaneous 
liquefaction by expansion M. Claude had brought his process of recov- 
ering the energy of expansion down to a practical basis. With ma- 
chines utilizing an exterior air compression capacity of 75 horse- 
power the yield of this process becomes as high as 0.7 liter of liquid 
air per horsepower-hour. 
This idea can be carried still further, however. The air which 
arrives at the auxiliary motor at a pressure of 40 atmospheres and 
a temperature of about —100° C. can be subjected in expansion to a 
too great drop in temperature. To avoid the recurrence here of the 
difficulties encountered in the original process all that has to be done 
is to carry out the expansion by degrees in several auxiliary cylinders. 
The air of the first expansion can circulate about a first liquefier, into 
which is led a deflected portion of air from the feed circuit in a cold 
and compressed condition. The circulating air is warmed up and 
goes on to be expanded in a second auxiliary cylinder. This air from 
the second expansion is sent into a second liquefier similar to the 
first, and is finally led into the economizer. In practice the two 
liquefiers are not distinct; the two currents of air after expansion 
merely circulate about different sections of the same liquefaction 
apparatus. M. Claude has given the name of “compound liquefac- 
tion” to this last process. It marks a new and important step in the 
technique of the liquefaction of air. In machines of the type 
described above the yield of liquid air, by the application of com- 
pound liquefaction, is as high as 0.85 liter per horsepower-hour. 
