374 
of less 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 
