428 Prof. Hennessy on the Vertical Currents 
of the mercury were observed. In thermometers freely exposed to 
the air, the mercury sometimes rose or fell three degrees Fahren- 
heit in three minutes. The longest fluctuations did not occupy 
more than six minutes. The fluctuations diminished the more 
the thermometers were protected from the influence of the cur- 
rents of air. 
A further confirmation of these results is found in the Report 
of the Director of the Radcliffe Observatory at Oxford, relative 
to the meteorological observations during the year 1857. 
The thermometrical curves exhibited a remarkable serration 
during the day-time of the most brilliant months of the year. 
This serration entirely ceased during the winter, and on gloomy 
days at every season: its intensity seemed to increase with sun- 
shine. It is readily explained by the action of small atmosphe- 
rical currents alternately ascending and descending, the former 
producing a sudden and brief elevation of the mercury, and the 
latter a sudden and short depression. The curves referred to 
were obtained at the Radcliffe Observatory, by a very beautiful 
application of the waxed-paper photographic process; and the 
results here noticed would probably never have been exhibited 
by the ordinary observations at stated hours. I cannot refrain 
from remarking that the success which has attended this portion 
of the application of photographical registration to meteorology, 
has much increased my confidence in its trustworthiness, while 
it has inspired a feeling of deep regret at the loss which science 
has sustained by the death of Mr. Johnson, to whose able 
management and indefatigable labours these and many other 
results are mainly due. 
7. To such small currents we may attribute whirlwinds of 
more or less magnitude, from those which we often observe on 
dusty roads, to the grand and frequently dangerous phenomena 
of the desert. Mr. Belt, who writes in the Philosophical Maga- 
zine for January 1859, presents some very instructive observa- 
tions on this subject. The ascending currents over dry ground 
in the interior of Australia, were frequently observed by him to 
carry leaves and dust to the upper regions of the atmosphere. 
Often, when travelling over parched plains, this observer saw the 
air quivering over the hot ground as if close to the wall of a fur- 
nace; suddenly a miniature storm arises, and after a few minutes’ 
violence it as suddenly ceases, while the quivering of the air is 
no longer seen and the atmosphere does not feel oppressive. All 
these phenomena are obviously the results of more or less ener- 
getic interchanges between masses of air possessing different 
temperatures. The process of convection in this case is not of 
a gentle and gradual nature, but takes place with fitful violence. 
The phenomena here referred to seem to present on a small 
