16 ANNIVERSARY ADDRESS. 
is now to be measured ; and there is some difficulty in the way, 
since the weight of the cardboard would vary with the state of 
the weather, and this would be a serious difficulty if the sun’s 
effect is to be ascertained by weighing. 
In connection with this subject, it will be remembered that 
about seven years since a French inventor, M. Mouchot, exhibited 
before the Emperor a steam-engine worked by direct sun heat. 
He has been working at the machine ever since, and some of his 
recent results are so good that it seems probable that direct sun 
heat may in warm, fine countries become an economic source of 
power. . 
The machine is thus constructed :—A mirror in the form of a 
truneated cone, 74 inches in diameter, has its axis converted into 
a boiler 11 inches in diameter and 31 inches high. With this 
the mirror makes an angle of 45°, so that all the rays falling on 
it are reflected into the boiler. To prevent loss of heat by 
radiation, the boiler is covered with an air-tight glass case. This 
apparatus is placed on an axis parallel to that of the earth (a 
polar axis) and then turned so that the axis of the cone points to 
the sun. Clock-work is then connected to the polar axis, and 
keeps the mirror turned to the sun. The results obtained in 
ordinary weather are as’ follows :—4 gallons of water introduced 
at a temperature of 68° Faht. at half-past 8 a.m. were turned 
into steam of 380 lbs. pressure per square inch in forty minutes. 
The pressure was then allowed to rise to 75 Ibs. on the inch, 
which it did in a few minutes, but the boiler was not strong 
enough to carry the test toa greater pressure. The steam was 
used to drive an engine and a pump. At another trial the 
machine distilled a gallon of wine in fifteen minutes. 
These are surprising results to be obtained in the climate of 
France, and seem to make the experiment. worth trying on our 
sunny plains. 
Another use, however, might be made of it. Such an instru- 
ment, properly constructed, might be kept at work always, and 
made to deliver the water distilled into measures placed at fixed 
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