i88i.] 
219 
The Radiograph and its Uses . 
is duration of action, i.e ., the number of hours during which 
the sun actually shines. This quantity is given by the 
length of the day less such time as it is obscured by cloud 
or fog. 
The second quality is intensity. Everyone must have 
noticed that the efficacy of the sun’s rays is very much 
greater in the middle of the day than in the early morning 
or late evening, and also greater when the sky is clear and 
the air transparent than when the one or the other is 
dimmed with haze or smoke. 
Instruments are already in existence which register one of 
these factors without the other, and consequently give an 
imperfedt answer to the question. Thus there is the so- 
called vacuum solar radiation thermometer which is in 
adtion in some meteorological observatories. It is a self- 
registering maximum thermometer with a blackened bulb, 
enclosed in an outer case of thin glass, exhausted as far as 
possible of atmospheric air. This instrument shows doubt- 
less the greatest intensity of solar radiation reached, but it 
gives no sign whatever as to whether this maximum has 
prevailed for ten minutes or for five hours. 
On the other hand, there is an instrument which has ob“ 
tained a kind of official recognition which records duration 
without giving any accurate notion of intensity. It consists 
of a burning glass which concentrates the sun’s rays upon 
a band of paper moved by clockwork at a regular rate. 
When the sun is shining the paper is burnt to charcoal in a 
continuous line. As soon as a cloud comes over the sun the 
adtion ceases. In consequence of the movement of the 
paper it is possible to observe during how many hours and 
minutes this carbonising adtion has been going on. Whether 
the sun has shone with just sufficient intensity to scorch 
the paper, or with twice that intensity, it cannot be made to 
appear. 
As an improvement on this rude contrivance Mr. D. Win- 
Stanley, F.R.A.S., lately of Blackpool and of Paris, but 
now of Richmond, has devised the instrument of which we 
give a figure. 
The instrument consists essentially of a firmly fixed and 
sensitive balance, carrying a mercurial thermometer, or 
rather thermoscope, fitted at each end with a glass bulb 
(T t') containing air. The former of these bulbs alone is 
exposed to the sun : A B are balls for adjusting the equilibrium 
of the balance, and c d are stops which prevent it from tilting 
too far. When the sun shines on T the air within expands, 
and drives the mercury to the right along the horizontal 
Q 2 
