21 
Ihese causes of variability I have attempted to get rid of 
in the following manner. With the help of Mr. Jordan, 
mechanician at Owens College, the following instrument lias 
oeen constructed. It consists of a large mercurial ther- 
mometer with its bulb in the middle of a cubical cast iron 
c namber, this chamber being of such massive material that 
i s temperature will remain sensibly constant for some time. 
The chamber with its thermometer has a motion in azimuth 
round a vertical axis A, andtdso a motion in altitude round 
a horizontal axis B. A three inch lens C of 12 inches focal 
length is attached by means of a rod to the cubical chamber 
so as to move with it. The nature of this attachment will 
be * een m the figUre - Tllus the whol e instrument may be 
easily moved into such a position that the lens as well as 
ie upper side of the chamber which is parallel to the plane 
o the lens may face the sun, and an image of the sun be 
nown through a hole D in the side of the chamber upon 
the thermometer bulb E. 
