370 SirW. Snow Harris on the Laws and Operation [Jane IG, 



of the index over the graduated arc m either direction, and is eventually 

 counterpoised by the elevation or depression in the water of the hydrostatic 

 cylinder suspended from the opposite side of the wheel. The divisions on 

 the graduated quadrant correspond to the addition of small weights to 

 either side of the balance, which stand for or represent the amount of 

 force between the attracting plates at given measured distances, wdth given 

 measured quantities of electricity. This arrangement is susceptible of very 

 great accuracy of measurement. 



The experiment requires an extremely short time for its development, 

 and no calculation is necessary for dissipation. The author carefully de- 

 scribes the manipulation requisite in the use of this instrument, together 

 with its auxiliary appendages. He considers this electrometer, as an in- 

 strument of electrical research, quite invaluable, and peculiarly adapted to 

 the measurement of electrical force. 



14. Having fully described this electrometer, and the nature of its indi- 

 cations, certain auxiliary instruments of quantitative measure, to be em- 

 ployed in connexion with it, are next adverted to. 



First, the construction and use of circular and globular transfer measures 

 given in the preceding Table, by which given measured quantities of elec- 

 tricity may be transferred from an electrical jar (charged through a unit- 

 jar from the conductor of an electrical machine) to any given surface in 

 connexion with the electrometer. The electrical jar he terms a quantity - 

 jaVy the construction and employment of which is minutely explained ; as 

 also the construction and employment of the particular kind of unit-jar he 

 employs. 



15. Two experiments (1 and 2) are now given in illustration of this 

 method of investigation. 



Experiment 1 developes the law of attractive force as regards quantity ; 

 which is found to vary with the square of the number of circular or glo- 

 bular inches of electricity, transferred to a given surface in connexion with 

 the fixed plate of the electrometer, the distance between the attracting sur- 

 faces being constant. 



Experiment 2 demonstrates the law of force as regards distance between 

 the attracting surfaces, the quantity of electricity being constant ; and by 

 which it is seen that the force is in an inverse ratio of the square of the 

 distance between the attracting plates, the plates being susceptible of per- 

 fect inductive action. From these two experiments, taken in connexion 



with each other, we derive the following formula, F a ; calling F the 



force, Q the quantity, and D the distance. It is necessary, however, to 

 observe that this formula only applies to electrical attractive force between 

 a charged and neutral body in connexion with the earth, the two surfaces 

 being susceptible of free electrical induction, both direct and reflected. 



16. The author now refers to several experiments (3, 4, 5, and 6), show- 

 ing that no sensible error arises from the reflected inductive action of the 



