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take place, wearing of commutators, drying of oil, burning of mercury and so on all tend to cause under-running rather than the reverse.
Failure to start is a much greater nuisance. Of course the alarm is promptly given in this case, but a surfeit of bellringing, especially during the night, can become very exasperating. Trouble arises as follows : Owing to circumstances previously mentioned the machine under-runs a trifle. This means that it stops at a point where the shutters instead of being absolutely open are a trifle behind their correct position. The motor now, when it starts, has not merely to let the shutters drop but first to pull them up a trifle, by far the heaviest phase of the whole series of events. Sooner or later the motor stops in its least efficient position, and fails to start with the weight of the shutters against it.
This trouble has been practically eliminated by a simple device. On the countershaft to which the motor is coupled instead of a plain pulley wheel there is a light clutch. To the same countershaft is coupled a gyroscope which as acquired sufficient power to push things a trifle farther and open the clutch. When therefore, the circuit is next made the motor has nothing to do but to reverse the clutch, and having started it will of course continue. To counteract the upward growth of the plant which would tend to carry the point of interest outside the sarea of the picture, mechanical stages are made capable of both lateral and vertical movement. These stages, which we term « Stats » are automatically controlled by small power units, practically portable replicas of the ordinary timing machines. The pinwheels of these control units are set so that they operate halfway between the taking of the pictures, thus avoiding any visible vibration in the case of a fragile plant. Previous observations are made to ascertain the amount of growth likely to take place, and the control unit adjusted accordingly. Of course in the case of root growth the apparatus is arranged so as to give a rising movement.
The photographing of the growing root is a particularly uncertain phase of botanical research, for whereas a few can be almost relied upon to produce excellent results, the majority are difficult, and some have, up to the present defied all efforts to record them. The main principle is as follows. Suppose we have a brick of soft clay, split it from top to bottom and charge the two cut faces with peat dust. We replace the parts, and sow a row of peas along the edge of the cut, covering them with wet fabric. The roots will drive downwards, generally taking the line of least resistance, and by disassociating the clay we can observe and photograph progress. The first films of root growth were made with such clay supports, but the method was cumbersome and except in the case of rapidlv growing and easy subjects the keeping of the clay in the correct condition of moisture was not at all a simple matter. More recently porous bricks have been prepared containing cavities into which suitable soil mixtures could be compressed, and