66 
its texture, floats before the sun, and occupies but three or 
four seconds in its transit, its presence, the duration ot 
its passage, and the degree of thermal obscuration it eflects 
are at once set down. 
The cylinder of the radiograph passes over a space of 
•875 of an inch per hour, a somewhat open range, but, as 
will be seen on reference to the tracings, the needle often 
moves for some considerable distance in both directions 
along the same thin line, thereby showing a practical 
instantaniety of action under very ordinary thermal changes 
in the radiance from the sky. The influence of the sun’s rays 
at daybreak is almost always shown, for some minutes at any 
rate,'before the sun himself is seen, and occasionally it would 
seem even for hours before his time to rise. 
It is not, however, now my purpose to dwell upon the 
interesting changes which take place in the intensity of the 
thermal radiance from the sky, my present object being 
to describe an instrument by means of which they may be 
recorded or observed. Doubtless in several of its details 
the ‘D’adiograph” may be improved, notably in the condition 
of its bulbs, and it would unquestionably be better if it 
computed for itself the areas included by its curves. This, 
I dare say, I shall presently enable it to do. Meanwhile, as 
a recorder of the duration and intensity of radiant heat the 
instrument, so far as I have seen, is the only one whose 
readings are uninfluenced by the temperature or the pres- 
sure of the air. 
