fi56 Royal Society. 



later observations. From a comparison of his own with the observa- 

 tions of the Rev. James Pauil, minister ofTuUy-nestle, he infers that the 

 heiglit of the particuhir aurora which was seen by them on the 20th, 

 did not, at its upper extremities^ exceed 4000 feet above the ground; 

 and is led to the general conclusion, that the aurora borealis is situated 

 in the region immediately above the clouds, and therefore varies 

 much in height, according to the different states of the atmosphere. 

 He believes it to be an effect of the development of electricity from 

 the condensation of vapour. The position of the fringes which 

 are constantly at right angles to the magnetic meridian, their pro- 

 gressive movements from the north magnetic pole, and their influence 

 on the needle whenever they come into the plane of the dip, are all 

 of them circum.stance.s which establish the relation of this phsenomenon 

 to magnetism ; while they at the same time illustrate the intimate 

 connection subsisting between magnetism and electricity. 



On the production of regular double Refraction, in the Molecules 

 of Bodies, by simple Pressure; with Observations on the Origin of 

 the doubly-refracting Structure; by David Brewster, LL D. FR.S. 

 Lond. & Ed. The author has already shown, in former papers which 

 have appeared in the Philosophical Transactions, that the phsenomena 

 of double refraction may be produced artificially, by effecting certain 

 changes in the mechanical condition of hard and of soft bodies. In 

 all these cases, he observes, the pheenomena are entirely different from 

 thoseof regular crystals; and in none of them is the doubly-refracting 

 force a function of the angle which the incident ray forms with one or 

 more axes given in position. In the year 1815 he noticed the de- 

 polarizing properties of a thin film of a mixture of resin .and white 

 wax, compressed between two pieces of glass. Accidentally meeting 

 with the specimen which had originally been the subject of this observa- 

 tion, he found that after fifteen years it still retained this property of 

 depolarization, and was induced to pursue the inquiry to which it led. 

 He varied the proportions of the ingredients, and observed in the dif- 

 ferent cases the modifications produced in the phsenomena by employ- 

 ing various degrees of pressure. He found that in every point there 

 existed an axis of double-refraction perpendicular to the plane of the 

 film ; and that the doubly-refracting force varied with the inclination 

 of the incident ray to this axis ; just as happens with all regular uni- 

 axal crystals. He infers from his observations, that the property of 

 uni-axal double-refraction is communicated to the molecules simply 

 by the agency of pressure ; for in all cases where pressure has not 

 operated, the aggregate does not exhibit this pro])erty. These effects 

 are precisely the same as those which would be produced by sub- 

 jecting elastic spheres to a regular compressing force ; the axis of 

 pressure becoming an axis of positive double refraction ; while ex- 

 tension, on the contrary, produces a negative axis. 



From the consideration of the preceding facts, the author is led to 

 a very simple explanation of the origin and general phenomena of 

 double refraction in regular crystals. He considers this property as 

 not being inherent in the molecules themselves ; but as resulting 

 from their compression, either by an extraneous force, or by their 



power 



