A quartz glass sphere, 4" (10.16 cm) in diameter, mounted on top of a 7' (~2.2 m) stand. As sunlight passes through the sphere, it becomes focused and burns a "line" through a piece of treated paper which is positioned beneath the quartz glass sphere.

Any break in the "burn line" is indicative of cloud cover. The treated paper is marked in such a way that the time of cloud cover occurrence can be determined. There are 3 positions in which to place the "solar cards". One is specifically used for summer months only. Another is used only for Winter months. The 3rd position is used only during spring and fall. Different positions of the "solar cards" are to compensate for changing sunlight-to-Earth angles throughout the year.

Above are 4 different pieces of treated paper. Each paper above represents one 12-hour day.
The Roman numerals on the paper denote the local time (XII is 12 noon, III is 3pm, etc.)
Notice the burn marks. These burn lines (or dots) represent times when sunshine occurred at
our station. These are papers that have been used at our station.

The top paper was used on an overcast day (no burn lines).

The second paper from the top was a mostly overcast day with only a few instances of sunlight (burn lines show sunlight occurred for just a couple minutes around 9:50am, then for about 10 minutes around 10:30am, and a couple minutes around 11:35am. If you look all the way to the right on the second paper, you'll see a small burn showing that there was sunlight for just a minute around 4:40pm.

The third paper down from the top shows a day with scattered clouds. Notice burn lines indicating sunshine from about 7:20am to 8:45am, then again for 10 minutes around 11am, again for a couple minutes around 11:30am, then again from around 12:30pm to just after 1pm, then again from about 4:10pm to about 4:40pm, and finally a minute of sunshine at about 5pm.

The bottom paper shows a day when we had sunshine throughout most of the day. Notice that the burn line is unbroken, indicating solid sunshine from about 7:45am to 4:45pm. Also notice the intensity of the burn line.

Our station may be one of the only stations left in the country that has a Duration-of-Sunlight-Recorder! Before the days of pyrometers, this device (and a human observer), were the only ways to record the day's duration of sunlight.