648 REPOET — 1892, 



tbrougb whicli a current of electricity can be sent. Below this cylinder is placed 

 another provided with a copper disc at the top, this cylinder being kept cool by 

 the passage of a stream of cold water through it. Between the two copper discs- 

 comes the layer, about 1 mm. thick, of the liquid the conductivity of which is to be 

 determined. The temperatures of the copper discs are determined by means of 

 thermo-j unctions. A current is sent through the heating coil till the temperature 

 of the heating cylinder, as indicated by a thermometer in it, is about 5° above that 

 of the cooler. By adjusting the current through the coil this temperature excess 

 can be maintained for any required time. The temperatures of the copper discs 

 are during this time determined, and from their difl'erence and the amount of 

 energy supplied by the current to the heater, it would be possible, if there were 

 no loss of heat due to radiation, to determine the thermal conductivity of the 

 liquid between the discs. By another experiment this loss can, however, be deter- 

 mined and subtracted from the total loss, and thus the conductivity found. In 

 another form of the apparatus this correction is got rid of by surrounding the 

 heater with an annular vessel, separated from it by a layer of non-conductor. 

 This outer vessel is also provided with a heating coil, and by means of rheostats 

 in circuit the temperature of both heaters, as indicated by thermometers in them, 

 is kept the same. The energy supplied to the inner heater is then merely that 

 necessary to compensate the loss of energy due to conduction through the layer of 

 liquid at the base of the cylinder, so that the conductivity of the liquid is readily 

 found from the observations. 



5. Interim Report of the Committee on a National Physical Laboratory^ 



A Discussion on a National Physical Laboratory was opened by 

 Professor Oliver J. Lodgk, F.B.S. 



7. On Graphic Solution of Dynamical Problems. 

 By Lord Kelvin. 



The method of drawing meridional curves of capillary surfaces of revolution,, 

 described in ' Popular Lectures and Addresses,' vol. i., 2nd edition, pp. 31-42, and 

 illustrated by woodcuts made from large scale curves, worked 

 ^^''* '■■ out according to it with great care by Professor Perry when 



a student in the Natural Philosophy Class of Glasgow 

 University, suggests a corresponding method for the solution 

 of dynamical problems. 



In dynamical problems regarding the motion of a single 

 particle in a plane, it gives the following plan for drawing 

 any possible path under the influence of a force of which 

 the potential is given for every point of the plane. Suppose, 

 for example, it is required to find the path of a particle pro- 

 jected in either direction, with any given velocity, in any 

 given direction through any given point P^, (fig. 1). Calcu- 

 late the normal component force at this point ; and divide 

 the square of the velocity by this value, to find the radius 

 of curvature of the path at that point. Taking this radius 

 on the compasses, find the centre of curvature, C,,, in the 

 line, PqK, perpendicular to the given direction through Pq, 

 and describe a small arc, Pj,P,Qj, making PjQi equal to 

 about half the length intended for the second arc. Calculate 

 the altered velocity for the position Qj, according to the potential law; and, as 

 before for P^,, calculate a fresh radius of curvature for Q, by finding the nonnal 

 component force for the altered direction of normal and for the velocity corre- 



