466 BOTANICAL GAZETTE [ DECEMBER 
should be kept in perfect order. It is especially necessary that 
the pulley bearings be clean and carefully adjusted, and that the 
guide wires for the needle carriage be kept polished. As the 
machine is intended to record increments of growth too small 
to be seen by the unaided eye, it is obvious, moreover, that it 
should be kept free from tremor or jar. It must, in fact, be 
treated as a machine of delicate precision in order to secure 
results of the highest value. 
Centrifuge.—This machine (f/. XXV) is for the purpose of 
applying to small plants a force, in connection with or independ- 
ent of gravitation, that shall induce a strain within the organ of the 
same physiological nature as that brought about by gravity. Adisk 
of cork 10™ in diameter, to which germinating seeds may be pin- 
ned is made to revolve at considerable speed in a moist chamber. 
The moist chamber consists of a shallow metal pan, into which 
a deep glass cover is fitted. The bottom of the pan is pierced 
by two openings, one of which conducts a constant supply of 
water into the chamber through a rubber tube terminating in a 
bent glass tube within. The glass tube can be adjusted from 
without so as to permit the water to fall in drops upon any part 
of the disk. An opening at the opposite side of the pan takes 
away the surplus water through a rubber tube. 
The revolving disk is turned by a small electric motor, sUp- 
plied with current from two Edison-Lalande cells of type Q. 
These batteries are the most satisfactory for small motors yet 
devised. When setting them up it is best to put the jar into a 
vessel of water, and let it remain there until the potash has fully 
dissolved and the excessive temperature abated, to avoid break- 
ing the jar. After the cell is in readiness it requires no further 
attention until exhausted, when the potash, oil, zincs and carbon 
must be replaced with new ones. Although this is by far ~ 
best motor battery known, yet, like all batteries, it is likely to 
prove treacherous at times. Where suitable water power °F 
other more reliable motive power is available, it should be 
adopted. 
When the cork disk is whirled horizontally the centrifugal 
