Birge—A Second Report of Limnological Apparatus. 543 
the sun is received by 20 iron-constantan thermal couples, whose 
junctions are covered by blackened silver disks about 1.5 mm. in 
diameter. They are arranged in two series of ten each, making 
a double row about 20 mm. long and 3 mm. wide. They are cov¬ 
ered by a hemispherical glass dome about 65 mm. in diameter, 
which is cemented into the top of a brass box about 75 mm. square 
and 20 mm. thick. This box is made of two pieces of brass closely 
fitted together so as to exclude moisture. In a receptacle on one 
side of the box is placed some phosphorus pentoxide, so that all 
moisture may be removed from the air within the box. From 
one side of the box leads a tube about 18 cm. long, into which are 
sealed the connections of the wires from the thermal couples and 
the wires of the cable leading to the galvanometer. 
This box containing the thermal couples is mounted on a frame 
46.5 cm. long and 15.5 cm. wide, made of solid brass bars 12.5 mm. 
square. The frame is therefore heavy, weighing about 2 kg. It 
is made long so as to carry the lines by which it is suspended in 
the water to such a distance that their shadow will not be likely 
to fall on the thermal couples. It is made heavy so that its 
equilibrium will not be disturbed by opening and closing the cover 
of the thermal couples in order to admit or exclude the sun. This 
cover is hemispherical and made of thin brass; it turns on an 
axis attached to the frame and is operated by a pinion on the axis 
and a rack which slides on one of the longer sides of the frame. 
The rack is pulled back and forth by two cords, one attached to 
each end. These pass through small pulleys on the frame and so 
lead up through the water to the boat. Two small spirit levels 
are attached to the frame at right angles to each other so that 
the instrument may be held in a horizontal position. When in 
use this receiver is suspended from a small braided rope. From 
each corner of the frame a strong cord about 75 cm. long extends 
to the rope. Near the upper end these cords pass through a brass 
tube about 5 cm. long. This is fastened to the cords, which are 
cemented into it, and on each side is attached an eyelet through 
which passes one of the cords for operating the shutter. This ar¬ 
rangement prevents these small cords from becoming entangled 
with the various parts of the receiver and frame or twisted with 
the suspension cords. 
