864' 



and of a pair of magnets placed symmetrically, and parallel to the 

 axis of the suspended coil ; thus we have the relative intensities of 

 the induced currents, and the intensities of the inducing forces are 

 obtained by observing the deflections w^hich they produce on that 

 coil, excited by passing a feeble current through it. 



4th. The instrument may be applied in conjunction with the rheo- 

 meter, to determine the intensity and duration of instantaneous cur- 

 rents. In the latter instrument the moment of rotation is as the 

 intensity ; in the former as its square. The angular velocities gene- 

 rated in them are as these moments and the time ; but also as the 

 times of oscillation inversely, and the extreme swings of each di- 

 rectly. It follows therefore that we have two equations from which 

 the intensity and the time can be determined. He illustrates 

 this by measuring the time of the discharge of a Leyden battery 

 through a moistened cord, and points out other applications of im- 

 portance. 



5th. The instrument differs from the rheometer in not being 

 affected by the direction of the current ; it therefore will measure 

 currents whose direction changes rapidly, and which therefore w^ould 

 produce no visible effect in the rheometer, as they neutralize each 

 other's action in it. Such alternation currents may exist ; they would 

 be a vibration of the electricity, not a progress of it. He suggests 

 that the undulations of light might produce effects of this kind, and 

 gives as an example the action of the sound waves : a bar of steel 

 properly arranged being made to sound, deflections of twenty or 

 thirty divisions of the scale are produced. To go into the develop- 

 ment of the theory by which he deduces, from axioms given by ob- 

 servation, the laws of electrodynamic action, would occupy too much 

 time ; and I will only add, that he generalizes Ampere's law, and 

 transforms it into one depending on electric mass, velocity and di- 

 stance. PVom this he deduces the theory of constant currents and 

 that of Vol ta- induction in all its cases. The slight account I have 

 given of Weber's researches, conveys but a very inadequate idea of 

 their importance ; a more detailed notice would have been unsuitable 

 to the present occasion. 



A most important contribution has recently been given by Pro- 

 fessor Dove to the science of Meteorology — his maps of the monthly 

 isothermal lines of the globe. In 1847 he communicated to the 

 British Association a table of the mean temperature of 800 stations, 

 for every month of the year, and to it he has since added 84 stations. 

 From this table he has constructed his maps. In the memoir which 

 accompanies the maps, he explains shortly, but clearly, his methods 

 of proceeding ; he had previously communicated them at length to 

 the Berlin Academy. He also deduces from the facts brought to light 

 by his extensive generalizations, new views of peculiar interest in 

 their bearing on the science of Climatology. To have separated the 

 annual mean heat received from the sun into its invariable monthly 

 proportions, or those of season, was, as it appears to me, a great step, 

 the greatest probably that has hitherto been made in the science of 

 Climatology. It affords the key to the scientific understanding of 



