yS Count Rumford's Account of a Method of 
opening the box. I said above, that the height of the vertical 
wing attached to each of the cylinders was equal to the height 
of the cylinder itself : — this must be understood to mean, not 
the total length of the cylinder, comprehending that part of 
it which passes into, and through the bottom of the box ; but 
merely its height above the bottom of the box, or part pro- 
jecting, namely 2-^ inches. 
As it is absolutely necessary that the cylinders should con- 
stantly remain precisely perpendicular to the bottom of the 
box, or parallel to each other, it will be best to construct 
them of brass, and instead of fixing them immediately to the 
bottom of the box (which being of wood may warp), to fix 
them to a strong, thick piece of well hammered plate brass, 
which plate of brass may be afterwards fastened to the bottom 
of the box by means of one strong screw. In this manner two 
of my best instruments are constructed. And, in order to se- 
cure the cylinders still more firmly in their vertical positions, 
they are furnished with broad flat rings, or projections, where 
they rest upon the brass plate ; which rings are ^ of an inch 
thick, and equal in diameter to the projection of the wing of 
the cylinder, to the bottom of which they afford a firm sup- 
port. These cylinders are likewise forcibly pushed, or rather 
pulled against the brass plate upon which they rest, by means 
of compressed spiral springs, placed between the under side of 
that plate and the lower ends of the cylinders. 
Of whatever material the cylinders be constructed, and 
whatever be their forms or dimensions, it is absolutely neces- 
sary that they, as well as every other part of the photometer, 
except the field, should be well painted of a deep black, dead 
colour. That will prevent the inconveniences which would 
