The instrument consists of a Liebig's condenser mounted equatorially, so as to follow the sun by one motion. Projecting from the upper part of it, and enclosing the steam tube, is the boiler, which is a tube of half an inch diameter, where it receives the sun's rays, and widens out to 1 inch diameter in the steam space. This steam space or dome is formed by an inverted test tube, so that the operation can be easily watched. The tube is surrounded by a concave reflector made up of three conical surfaces of silvered copper. The diameter of the outer edge of the reflector is 13·5 inches, and its effective surface is almost exactly 1 square foot. The length of tube on which the rays were concentrated was 2 inches, and as its diameter was half an inch, its surface was 3·14 inches, therefore the thermal magnifying power of the combination was 46. These proportions were chosen in some doubt as to their suitability, but their choice was justified by the result. The amount of water actually in the focal portion of the boiler at any moment was 5 cubic centimetres; as the steam passed down through a central tube, and was condensed by the water rising outside of it to supply the place of what was evaporated in the boiler, the supply or feed to the boiler was effected practically at the temperature of ebullition.