INTENSITY OE THE CHEMICAL ACTION OE TOTAL DAYLIGHT. 
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narrow strips like those used in Morse’s telegraph, and of sufficient length (about 
3 metres) to last for one day’s observations, is wound on to a bobbin (B, fig. 2), from 
which it passes over the light metallic wheel (W, fig. 2) about 15 centims. in diameter, 
to the circumference of which one end of the strip is made fast. The escapement (E, 
fig. 2) is connected with the keeper (K) of a small electromagnet (M), round which the 
current from the battery passes whenever contact is made by the clock at E, fig. 1. 
The escapement-wheel (F, fig. 2), worked by a spring (S), is fixed on to the axis of the 
wheel (W) carrying the paper, so that when a current passes through the electro- 
magnet the keeper (K) is attracted, the escapement (E) released, and the paper carried 
forward on the wheel to a distance regulated by the number of the teeth of the 
escapement-wheel (F). As soon as the current ceases, the keeper is brought back into 
the position shown in the drawing by means of the spiral spring at its extremity. 
When the minute-wheel (A, fig. 1) of the clock comes round to a given point, a fixed 
pin (jp) on this wheel presses against the long end of a lever (L), and this pushes down 
the elastic arm (E) (which is insulated from the rest of the clockwork) on to the thick 
platinum pins fixed on to the metal wheel (D). By the passage of the pins in front 
of the end of the elastic arm contact is made, and then a current passes from the clock 
to the electromagnet on the insolation-apparatus. The paper on the wheel is then 
suddenly moved forward such a distance as is necessary in order to expose a fresh 
portion under the circular opening (O, fig. 3) on the upper side of the metal cover of 
the insolation-apparatus. This small disk of paper (4 millims. in diameter) remains 
exposed to the daylight until removed by the movement of the keeper caused by the 
contact of the second pin at E, fig. 1 ; a fresh disk of paper then instantly becomes 
visible, in its turn to be removed after having been exposed for a known length of 
time. This successive exposure of disks of sensitive paper for increasing periods of 
time up to 30 seconds goes on until the wheel carrying the platinum pins has com- 
pleted one revolution. The fixed pin Q?) on the minute-wheel has during that time 
passed so far round, that it has now ceased to press upon the curved and thickened end 
of the lever (L), and the tip of the elastic arm (E) is now drawn back by a small insu- 
lated spring (S) ; so that it remains in this position until an hour has elapsed, when it 
is again pressed on to the platinum pins by the point on the minute-wheel, and the 
exposure of another set of disks of sensitive paper occurs again. During the space of 
nearly an hour, during which the wheel (W, fig. 2) remains stationary, a very dark 
disk is obtained ; and if one of these dark disks be marked during the day with the 
hour of exposure, the times of all the different exposed disks can be ascertained. 
When in use the insolation-apparatus is covered with a light tight blackened metal 
cover (C, fig. 3), having a thin piece of metal at the top, which, when the instrument 
is placed in position, lies horizontally. In the centre of this plate a circular opening 
4 millims. in diameter is bored, the edges of which are carefully bevelled off. A steel 
spring (A, B, fig. 2), over which the prepared paper passes, presses the strip against the 
lower surface of the horizontal metallic plate, so that the disk of paper is seen to lie 
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