MEASURING THE DIRECT HEAT OF THE SUN. 



17 



These causes of variability I have attempted to get rid of 

 in the following manner. With the help of Mr. Jordan, 

 mechanician at Owen's College, the following instrument 

 has been constructed. It consists of a large mercurial 

 thermometer 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 alti- 

 tude round a horizontal axis B. A three-inch lens C of 

 1 2 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. 



SER. III. VOL. VI, c 



