82 
tration of his rays •when the sun may chance to shine. 
During the past few years I have devoted some attention 
to this matter and devised a number of appliances having 
the same object for their end but differing materially both 
in their construction and in the manner of their use from 
the apparatus I have named. 
One of these, with your permission, I will now describe. 
It is an arrangement which places a lead pencil on a sheet 
of paper and writes down therewith when and for how long 
the sunshine lasts. 
It consists essentially of a differential thermometer with 
a long horizontal stem, in which latter is contained through- 
out the greater portion of its length some fluid intended to 
operate by its weight. This thermometer is attached to a 
scale beam or some equivalent device which also carries the 
pencil by means of which the record shall be made. 
The whole is so arrauged that in its normal state it rests 
gently — upon that side to which the pencil is not attached — ■ 
on an embankment provided for that end. 
Close beneath the pencil point a disc of metal rotated at 
the proper speed carries a paper dial whereon marks and 
figures are engraved corresponding with the hours at which 
the sun may shine. 
When using this instrument I have it enclosed within a 
box which permits one bulb only of the thermometer — 
that most distant from the clock — to be affected by the 
radiance of the sun, which when it shines expands the air 
contained therein, forces the fluid along the tube and by 
altering the equilibrium of the beam brings some portion 
of its weight to bear upon the pencil point, and so the record 
is commenced. 
When the sun becom.es obscured, the air expanded by his 
rays contracts, the fluid in the tube returns, the normal 
equilibrium is restored, and the pencil ceases to produce its 
mark. 
