204 Royal Society : — 



and coutrary to that which the rotation of the substance would give 

 if there were no twist. But the effect on the plane of vibration, due 

 to an angular velocity w, of rotation of the substance, is, as we have 



seen, one turn in — ~ wave lengths : and therefore it is one turn in 



8-; ^;- wave lengths when the angular velocity is — The effect 



of a twist amounting to one turn in a length, s, a small fraction of the 

 wave length, is to cause the plane of vibration of a wave to turn 

 round with the forward propagation of the wave, at a certain rate, 

 in the same direction as that of a point kept on one of the screw 

 surfaces. 



From these illustrations it is easy to see in an infinite variety of 

 ways how to make structures, homogeneous when considered on a 

 large enough scale, which (1) with certain rotatory motions of com- 

 ponent parts having, in portions large enough to be sensibly homo- 

 geneous, resultant axes of momenta arranged like lines of magnetic 

 force, shall have the dynamical jyroperty by which the optical phce- 

 notnena of transparent bodies in the magnetic field are explained ; 

 (2) with spiral arrangements of component parts, having axes all 

 ranged parallel to a fixed line, shall have the axial rotatory property 

 correspondiny to that of quartz crystal ; and (3) with spiral arrange- 

 ments of component groups, having axes totally unarranged, shall 

 have the isotrojiic rotatory profierty possessed by solutions ofsuyar 

 and tartaric acid, by oil of turpentine, and many other liquids, 



"On Quantitative Measurement in Statical Electricity, and on 

 some new Phsenomena of Electrical Force." By Sir William Snow 

 Harris, F.R.S. 



The author observes, that number, weight, and measure are the 

 foundation of all exact science, and that, as expressed by an eminent 

 and learned writer (M. Quetelet), no branch of human knowledge 

 can be held as being out of its infancy which does not in some way 

 or the other frame its theories or correct its practice by reference to 

 those elements ; he was hence led to seek and establish such rigorous 

 and exact quantitative processes in common electricity as wovdd mea- 

 sure the quantity of electricity in operation ; its attractive or repul- 

 sive force under given conditions, and its dynamical or current force 

 when traversing bodies under the form of electrical discharge. The 

 instruments which he has invented for this purpose have been all 

 honoured by a place in the ' Philosophical Transactions of the Royal 

 Society.' They amount to five in number, viz. the Unit Measure, 

 the Balance and Hydrostatic Electrometers, the Thermb-electrometer, 

 and the Bifilar Balance. In referring to such of these instruments 

 as are employed in the present research, the author briefly adverts 

 to their general construction, including the latest and best form 

 under which they have been placed. 



In the measurement of quantity, he considers the unit measure as 

 being the best and most accurate means of estimating quantity as yet 



