656 
ME. H. E. EOSCOE ON A METHOD OE MEASUEING- THE 
ordinary manner the hourly intensities which have been recorded on the paper during 
the day. 
This self-recording arrangement, though at first sight simple enough, involves points 
which have rendered its successful completion a somewhat lengthy and difficult matter. 
Thanks, however, to the skill of Mr. Charles Jordan, of Manchester, these mechanical 
difficulties have now been overcome, and the instrument perfectly answers the desired 
end. 
Owing, in the first place, to the great variations which occur in the chemical inten- 
sity of total daylight in different places, at different times of the day, and in different 
periods of the year, and, secondly, owing to the fact that, in order to be able accurately 
to estimate chemical intensity, the coloration acquired by the paper must reach, but 
not much exceed, a given tint, it becomes necessary, on each occasion when an obser- 
vation is needed, that the sensitive paper should be exposed mechanically, not once, but 
for several known but varying intervals of time, quickly succeeding each other ; so that 
whatever may be the intensity of the total daylight (supposed during those intervals to 
remain constant), some one at least of the several exposed papers will possess the requi- 
site shade. This is accomplished by a duplicate arrangement of a clock and insolation- 
apparatus. The clock has connected with its minute-wheel (Plate L. fig. 1, A) a train 
of three wheels (B, C, D, fig. 1), so arranged for speed that the last wheel (D) revolves 
once every two minutes. On the periphery of this metal wheel are fixed eleven stout 
platinum pins (marked 1 to 11, fig. 1), each projecting about 3 millims. from the face 
of the wheel. As this wheel revolves, each one of the pins is in turn brought for an 
instant into metallic contact with the platinum face of the elastic metallic arm (E, fig. 1). 
As the wheel passes on, metallic contact between the wheel and the arm is interrupted 
until the next pin comes into position. These platinum pins are so placed on the 
wheel that by its uniform rotation contact is instantly made, broken, and, after a given 
interval, again made and broken, in all eleven times in succession. The intervals 
during which contact is broken are of different lengths, dependent upon the positions of 
the pins. For use in this country the intervals are so arranged as to break the contact 
for times approximating to the following number of seconds : — 
Interval . . 
No. 1. 
No. 2. 
No. 3. 
No. 4. 
No. 5. 
No. 6. 
No. '7. 
No. 8. 
No. 9. 
No. 10. 
Seconds . . 
2 
3 
4 
5 
7 
10 
12 
17 
20 
30 
When the instrument is to be used in situations where the chemical intensity is much 
greater or much less than in our latitudes, different intervals must be adopted. Whilst 
the wheel is in metallic contact with the elastic arm (E), a current from four cells of a 
Bunsen’s battery (or if a slow and constant current is required twelve to twenty cells 
of a Le Clench) passes by means of a second elastic arm (F) through wires connecting 
the clockwork with the insolation-apparatus ; but this current ceases to pass as soon 
as the circuit at E is interrupted. The paper of constant sensitiveness, cut into long 
