21 
These 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 has 
been constructed. It consists of a large mercurial ther-= 
mometer with its bulb in the middle of a cubical cast iron 
chamber, this chamber being of such massive material that 
its temperature will remain sensibly constant for some time. 
The chamber with its thermometer has a motion in azimuth 
round a vertical axis A, and also 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 seen in the figure. Thus the whole instrument may be 
easily moved into such a position that the lens as well as 
the upper side of the chamber which is parallel to the plane 
of the lens may face the sun, and an image of the sun be 
thrown through a hole D in the side of the chamber upon 
the thermometer bulb E. 
