316 B. Silliman, Jr. and Chas. H. Porter 
mounted upon a sliding stand to allow of adjustment at dif- 
ferent elevations. For convenient adjustment of the prisms at 
the angle of total reflection a slight motion of ¢ 
rotation is provided by the knerled heads e, fig. 1. 
The edges of the illuminated discs are brought 
is equivalent to ths of one per cent of the whole quantity. 
rior accuracy and neatness of the instrument here described 1 
ery obvious. e dark chamber and compensating eye-plece 
give to the discs of light upon the ground glass diaphragm, 4 
facility of compensation and of adjustment hardly inferior to the 
accuracy attainable by Babinet’s polarizing photometer, to which 
there are some objections needless to be dwelt on here. 
ments with the instrument.—The following trials were 
made in the city of New Haven on the last evenings of January 
1856, during very severe weather, upon two samples of coal gas. 
On the Ist and 5th evenings the coals used in cuenging the re- 
torts consisted of a mixture of 
bei Ist. eg Sth 
Fairmount (Maryland), 5,725 Ibs. 5,642 Ibs. 
Newcastle (English), 5,725 * 5,641 “ 
Hillsboro’ (New Brunswick), 750 “ 750 “ : 
12,200 “ + 12,083 “ = 24,233 lbs. 
ae. sted 
hy ERS EY 
