​This is a Belfort Model 5-3850 Pyranograph.  The label on it has a postal Zone Number, so I expect it is older than 1963.  It measures solar radiation with two bimetallic strips, one of which is blackened while the other is in the shadow of a polished shield.  The differential temperature between the two strips is proportional to the amount of solar radiation.  The differential temperature causes the pen to move.  It has a spring-driven clock and uses a Belfort 5-1050AW chart for weekly recordings.       
The chart has a scale width of 0 to 3 g-cal/sq-cm/sq-in.  It requires intense direct sunlight to work, it's not sensitive enough to respond to diffuse light.

Below is a manual for the Belfort 5-3850 Pyranograph (25 pages):

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