474 The Botanical Gazette. [November, _ 
counterweight and is adjustable, so that the weight may be 
set at any point along the beam. For recording evaporation, 
rium and the circuit broken. Further evaporation causes a 
repetition of the process. The beam is protected from ob- 
jectionable up and down swing by a dasher attached to the 
vertical stem supporting the scale pan and working in a cup 
of glycerin. 
The register is shown on the right of the plate. It is ex- 
actly the same as that used for recording rainfall. _ Prof. 
Marvin describes it as follows:? ‘The record cylinder is 
mounted upon a horizontal axis with the clock movement in- 
side. The cylinder makes one revolution in twelve hours. 
The mechanism giving motion to the pen consists of an elec- 
tro-magnet and armature similar to those on the weighing 
gauge, and of a notched wheel fixed upon the end of a screw 
having cut upon it both a right and left hand thread of coarse 
pitch, viz., three threads to the inch. A cylindrical sleeve 
slides upon this screw, being guided by a slender rod below 
and parallel to the screw. A slender spring, with the record- 
ing pen attached to its point, is connected adjustably to the 
sleeve by a double friction joint which enables the pen to be 
set with great facility, the friction holding it accurately and 
firmly where placed. 
‘‘The armature of the electro-magnet engages directly the 
teeth of the notched wheel upon the right and left hand screw 
in such manner as to Cause it to revolve tooth by tooth, al- 
ways in one direction, with each vibration of the armature. 
The sleeve moving upon the screw is fitted with a crescent- 
shaped attachment which enters the thread of the screw- 
When the sleeve is set with the crescent in the thread at one 
end of the screw, the pen will, when the screw is revolved, be 
moved to the opposite end, the crescent-shaped piece being 
then guided into the other thread, and thus, upon continued 
rotation of the screw, causing the pen to return again to the 
*U. S. Weather Bureau. Cir. E, Instrument Room. 
