422 Prof. Hennessy on the Vertical Currents 
of temperature resulting from the presence and absence of the 
sun. As it is now well established that the distribution and 
changes of temperature in these islands are dependent upon other 
influential causes besides the direct action of the sun*, we can- 
not, in general, expect to find in our climate a similar diurnal 
periodicity so distinctly defined as that observed in the centre 
and south of Europe. Here, as well as on the Continent, moun- 
tains are favourable to the production of inequalities of tempe- 
rature, moisture, and density among the aérial strata, which 
thus become liable to a multitude of disturbances, and especially 
to the action of vertical currents. It seems to follow, that in 
mountainous countries vertical currents have well-marked rela- 
tions with the changes of the weather. 
If, as usually happens, lakes exist among the mountains, the 
mysterious occurrence called the “ bore” is also thus explained. 
The circumstance that the suddenly-formed wave thus designated 
always proceeds from a side of the lake bordered by steep mouns 
tains, immediately suggests such an explanation. Although a 
similar idea has occurred to other inquirers, I may be permitted 
to refer to an instance where a demonstration was presented by 
met of the efficiency of vertical currents in producing the “ bore” 
on the surface of one of our Irish lakes. The fact that such a 
sudden wave usually preceded a change of the weather in the 
district surrounding the lake, led me to think that the study of 
the effective cause of the bore itself might become of importance 
in meteorology. But to do this, we should possess means for 
observing the actual direction and, if possible, the force of the 
atmospheric currents. 
2. Hitherto all instruments which had been employed for 
observing the wind were devised exclusively with reference to its 
horizontal direction and intensity, from the simple wind-vane to 
the most finished anemometer{. I have attempted to modify the 
ordinary vane so as to make it an indicator of the actual direc- 
tion of the current, both in altitude and azimuth. Instead of the 
fixed surface against which the wind impinges in ordinary vanes, 
I had a dise suspended at the tail of the vane capable of rotating 
on an axis perpendicular to the line of direction of the instru- 
* See Phil. Mag. for October 1858; also a letter from the author to 
Major-General Sabine, ‘‘ On the Influence of the Gulf-stream on the Win- 
ters of the British Islands,” Proceedings of the Royal Society, vol. ix. p.324, 
+ In a letter to the Rev. T. R. Robinson, D.D., of Armagh. See Pro- 
ceedings of the Royal Irish Academy, vol. vi. p. 279. 
{ Some time after the anemoscope had been devised, my attention was 
called by my friend the Rev. Dr. Robinson, to a passage among the notes 
to Dr. Darwin’s poem of the ‘ Botanic Garden,’ wherein the writer indi- 
cates such an instrument; but he seems never to have realized this idea, 
and the apparatus which he proposed was essentially different from mine, 
