184 MINNESOTA BOTANICAL STUDIES. 



to fit the bearings. The supports are made of brass and screwed 

 to a black walnut base, which is built of narrow strips to 

 prevent warping. 



The paper ribbon on which the record is obtained is two- 

 thirds of an inch in width and made in two lengths, one suffi- 

 ciently long to run four and the other eight days. Across the 

 upper surface of the paper ribbon is printed a series of lines 

 that divide the ribbon into hour spaces, which are numbered 

 consecutively. These hour spaces are so ruled that the time 

 of registration can be read to one minute directly from the 

 ribbon. 



A source of error arising from the fact that the paper as 

 it is wound on the first roller increases the circumference of 

 the roller, and causes the paper to move at an increased rate 

 as the paper continues to be wound up, is avoided by having 

 each successive hour- space longer than the preceding one. The 

 correction, however, is very slight on account of the thinness 

 of the paper used, and would amount to only six minutes at the 

 end of the eighth day. 



The time marker consists of a pen made of brass, and large 

 enough to hold an amount of aniline ink sufficient for two 

 weeks registration. This pen is attached by means of a rolled 

 brass strip to the armature of an electro-magnet, which is 

 hung on a hinge close to the base. Thus, as the armature 

 moves in response to the attraction of the magnet, or the pull 

 of a tension spring, the pen is drawn through a short hori- 

 zontal distance. The rod simply serves as a support to the 

 pen. The pen presses against the paper on the roller, and by 

 means of the milled nut it can be kept at any required pressure, 

 or can be withdrawn from the paper entirely when the latter 

 is to be removed or replaced. 



When the circuit is open the armature is held back by the 

 tension spring, and the length of the brass strip is so arranged 

 that the pen then traces a line near the right hand side of the 

 ribbon, as it is shown in the plate. When the circuit is closed 

 the armature is attracted and the pen is pushed to the other 

 side of the paper ribbon, thus making a short line at right 

 angles to the length of the paper. If the circuit is immediately 

 opened the time of registration is marked simply by a single 

 cross mark. If. however, the circuit remains closed for some 

 time a line is traced on the left side of the paper . 



In reading the record in this case the length of time elaps- 

 ing between two successive closures of the circuit, is indicated 



