680 
Proceedings of the Royal Society of Edinburgh. [Sess. 
§ 7. Description of an actual Paddle-Wheel Instrument: and of 
its Self-Recording Device for Registering the Readings. 
The apparatus (figs. 16 and 17) was designed so as to be as rigid as 
possible. It is constructed chiefly of flat wrought-iron bars 1 " x J" in 
section, welded or bolted together, lock-nuts being used on all the bolts. 
The base A is of wood strengthened by various iron bars, and it is elevated 
Fig. 16. 
on legs 7" high in order that it may be put astride the long base bar of a 
spectro-photometer with which it is to be used. The electric motor, which 
is out of sight in both figures owing to the paddle-wheel D fixed on the 
end of its axle, is bolted to a horizontal platform, attached to a vertical 
hoop H. This hoop turns about the pivots PP', which revolve in brass 
plummer blocks connected to the support S. This is made of a single 
welded piece, in shape like the letter D laid on its side U, and is provided 
with projecting feet, which are bolted to the base A. The paddle-wheel 
D consists of an aluminium disc thick, having a brass hub by which 
