Page:Researches on Irritability of Plants.djvu/41

18 a clock or a phonograph motor, according to what may be the requirement of the speed. The quarter-plate size (11 X 8 cm.) is convenient for record, as it is not too large for book illustration. The suspending thread passing over pulleys is wound round the winding-wheel of the clock. This wheel is provided with click and ratchet, which allow it to be wound without interfering with the axis of the clock. Thus winding of the wheel in the left-handed direction pulls up the recording-slide. The running-down of the clock then allows the slide to fall at a uniform rate. The various speeds found necessary for different records were such that the entire length of the plate, 11 cm., travelled past the recording-point in ·5 second, 6 seconds, 15 minutes, 1 hour, or 3 hours. The first two of these rates were obtained from a phonograph motor employing two different-sized wheels. The last three were obtained by attaching three different-sized wheels to the clock-axis which carries the minute-hand. In these slow rates the movement of the plate is quite uniform from the beginning, but when, as in the first two cases, this has to be dropped at a relatively high speed, a short time will elapse, equivalent at most to the first fourth of the plate, before it becomes quite uniform. Should uniformity of such movement be specially desired for the record, it must be commenced after passing this first fourth. But on account of the chronographic signals which accompany the record, this uniformity is not absolutely essential, for they give us the data from which the time-relations of the curve may be derived.

I may here refer to a few practical points with regard to the preparation of the glass for record and its subsequent fixing. In order to produce an even layer of smoke on the recording-plate, it is moved over the gas-flame from a bat's-wing burner; and this deposit of smoke will be improved if the gas has been previously passed through a jar containing a small quantity of benzine. After the record is taken, it is fixed by pouring carefully over it a dilute solution of canada balsam in xylol. It is