374 



of loss specific gravity than dry air. Thus the barometric 

 and hygrometric curves would be the inverse of each other, 

 the maxima of the one corresponding to the minima of the 

 other; and as the author had previously shown that the 

 hygrometric, thermometric, and electrometric curves were 

 in accordance, the barometric curve would be the inverse 

 of the thermometric and electrometric curves also. The 

 author remarked, that if this character of the horary oscil- 

 lations of the barometer in Ireland be confirmed by the ex- 

 periments of other observers, it will either lead to new 

 views 'of this phenomenon generally, or show that the 

 quantity of aqueous vapour existing in Ireland is so great 

 as to cause the horary barometric oscillations to present 

 themselves in a different form from that in which they are 

 recognised in drier climates. 



The author adverted, in the last place, to the hypothesis 

 of Priestley and Beccaria, — that the upper regions of our 

 atmosphere were the chief depositories of the electric fluid, — 

 an opinion which he conceived must fall, if the origin of 

 atmospheric electricity be due (as his experiments prove) to 

 the existence of vapour ; as these elevated parts of our at- 

 mosphere are far above the region of permanent vapour, 

 or even of vapour at all. 



Professor Mac Cullagh read a paper ^' on the Dynamical 

 Theory of crystalline Reflexion and Refraction." 



In a former paper, presented to the Academy in January 

 1837, and printed in volume xviii. of the Transactions, the 

 author had reduced all the complicated phenomena of 

 reflexion and refraction at the surfaces of crystals to the 

 utmost regularity and order, by means of a simple rule, 

 comprised in his theorem of the polar plane. This rule, 

 which was verified by its agreement with exact experi- 

 ments, he had deduced from a set of hypotheses relative to 

 the vibrations of light in their passage through a given 



