The left-hand circle, or discometer, was divided by nineteen hundred and twenty concentric circles, equidistant from each other.

The outermost, about twice as far from the centre as from the external edge of the mirror, was exactly equal to the Sun's circumference when presenting the largest disc he ever shows to an observer on Earth.

Each inner circle corresponded to a diameter reduced by one second.

By means of a vernier or eye-piece, the diameter of the Sun could be read off the discometer, and from his diameter my distance could be accurately calculated.

On the further side of the machinery was a chamber for the decomposition of the carbonic acid, through which the air was driven by a fan.