222 
[April, 
The Radiograph and its Uses. 
plays against a metal drum, made to revolve at regular 
speed by clockwork, and tightly covered with a piece of 
glazed paper which has been evenly smoked over at the 
flame of a tallow candle. As the rays of the sun fall upon B 
the air enclosed is expanded, and the mercury is driven 
towards A. In consequence the equilibrium of the wheel is 
disturbed, and the point of the needle makes a scratch on 
the fine coating of lamp-black. 
Fig. 3. — Winstanley’s Radiograph ; an Instrument which records continuously 
both the Duration and the Intensity of the Sunshine. 
The slightest cloud passing over the sun adts upon the 
needle, leaving a distindt tracing upon the cylinder, which, 
in consequence of its rotation, is referred to the exadl time 
at which it took place. Unlike the former instrument, the 
radiograph leaves an impression during the whole of the 
day. The feebler the light of the sun the lower is the line, 
as shown in the accompanying diagram. Here it appears 
that in the experiment thus recorded the radiation was 
greatest at noon, and declined then very rapidly for nearly 
an hour, rising afterwards almost to its former point. The 
