224 
The Radiograph and its Uses . 
[April, 
It must further be observed that the general temperature 
of the air and the barometric pressure do not affedf the 
instrument, which works solely by the difference of pressure 
in B and A — a difference due to solar radiation alone. It is 
scarcely necessary to add the caution that the instrument 
must not be placed where radiant heat from walls, rocks, 
&c., which have been heated by the sun, may fall upon B. 
The radiograph is enclosed in a copper case, out of the top 
of which the bulbs proje<5t, and are protected against rain, 
&c., by a bell of thin glass. 
It is remarked that the instrument gives slight signs of 
activity even before sunrise, and that the needle rises above 
what may be called the zero-line at midnight. This takes 
place with such regularity that it cannot be ascribed to 
accident. 
It may be obje<5ted that the radiograph apparently indi- 
cates perpetual sunshine. We must, however, remember 
that even during the passage of the darkest clouds, the sun, 
though invisible, has a somewhat greater adlion than when 
he is below the horizon. We can often feel his rays when 
we cannot point out his exaft position in the sky. 
We must pronounce the radiograph a most important 
addition to the working resources of the meteorologist. It 
will certainly prove of great value in a comparative study of 
the intensity of solar radiation at different latitudes and 
altitudes, as well as in different states of the atmosphere. 
We trust the invention will be adopted without delay in 
meteorological observatories. 
