16 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Physicists have demonstrated that in the layers of air above the earth 

 there is always present a definite pressure or potential of electricity, , 

 leaking continuously away to earth, and in so doing traversing the 

 tissues of the plants which form a mantle of vegetation over the ground. 

 In times of thunder weather this electric pressure may be so increased 

 that, unable to leak away rapidly enough by ordinary means, the 

 pressure may be released by an electric spark, the lightning flash, 

 passing from cloud to earth. 



The electric potential above the earth is usually positive in sign; 

 but when the air above the plants is charged artificially it may be 

 charged with positive or negative electricity. Since the experiments of 

 Lem STROM it has been customary to use positive electricity, as both 

 this investigator and later workers have stated that with overhead 

 positive charges the results are more marked, but Lemstrom's experi- 

 ments are the only systematic ones upon the point. In later experi- 

 ments the air above the plant has been raised to a positive potential 

 practically equivalent to that which obtains in nature just prior to a 

 thunderstorm. 



If then it is desired to charge the air above the plants with elec- 

 tricity, it is necessary to have an apparatus capable of generating the 

 electricity, and a system of conductors above the area to be electrified 

 that will enable th;e. electricity to be discharged into the air above this 

 piece of ground. 



The apparatus for generating electricity may take various forms. In 

 the earlier experiments some form of influence or friction machine 

 was used, and for experiments on a comparatively small scale where 

 it is not desired to expend much money upon the electrical apparatus, 

 this method is still used. In my experiments at Bristol I have used 

 a home-made machine, constructed for me by Mr. T. Clarke, of 

 Bishopston, Bristol, and it has proved quite satisfactory for my pur- 

 pose, but for continuous running, unattended, this method is not to 

 be compared with the Lodge-Newman method to be described later. 



Such an influence or friction machine can be mounted upon ball- 

 bearings and driven from a small motor. It needs to be housed in 

 a very dry shed, and it is almost essential to have available some means 

 of warming the machine. 



My own machine is mounted within a case of glass and mahogany, 

 the base of which is formed of a piece of corrugated iron. The lower 

 side of this can be heated by means of a gas jet beneath, and it is pos- 

 sible to work this machine in all weathers and maintain an effective 

 pressure upon the small system of wires connected with it. At times 

 one spends many vexatious hours in persuading the machine to start, 

 but when in good condition and running day after day, all that is 

 usually necessary is to start up the motor, the machine then self- 

 excites, and will run continuously for as long as required. The 

 insulation of such a machine falls off with time, and it is necessary 

 to have the glass plates revarnished. 



One difficulty I have found, is that the products from the gas jet 



