430 Scientific Intelligence. 
ments is also given. The meeting for 1896 is to be held at 
Liverpool and the president elected is Sir Joseph Lister. 
2. On the origin of thunderstorms.—A new theory of thunder- 
storms was advanced by Prof. Michie Smith in his paper before 
the British Association on Indian thunderstorms. His observa- 
tions, made at Madras, showed that sheet-lightning occurs there 
every evening during several months of the year, always in the 
southwest and near the horizon. Lightning phenomena in the 
morning occur, on the other hand, in the northeast. The phe- 
nomena consist of actual discharges between two clouds, or two 
portions of the same cloud, and are not reflections of distant 
lightning; they take place in the upper portions of low-lying 
cumulus clouds. Professor Smith attributes them to the clouds 
formed in the regions of still air at the meeting of the land and 
sea breezes, and has observed in these regions the simultaneous 
rise of two close parallel clouds from the edge of the cumulus; 
such clouds are scarcely distinguishable except with oblique illu- 
mination, and it is within, or between, them that the discharges 
occur. The time of their formation depends on the hour at which 
the sea breeze sets in, being roughly three hours later. The land 
breeze being dry and dusty is negatively charged, while the sea 
breeze is known to carry a strong positive charge; equalization 
of the electrical states of the clouds formed out of these will, 
therefore, give rise to lightning. Professor Smith referred to the 
iridescence or nacreous appearance of the edges of the clouds 
when rapidly sinking, and considered this effect to be due to the 
dust left behind by them. 
This paper gave rise to an interesting discussion, chiefly with 
reference to the origin of dust in clouds, and the source of their 
electricity. Mr. John Aitken pointed out that thunderstorms are 
most probably the effect, not the cause, of purifying the air. He 
gave instances of thunderstorms on several successive days, all of 
which left the air dusty and impure; eventually the air cleared, 
and no more thunder occurred. Professor Schuster alluded to the 
fact that twenty-five theories of thunderstorms had been put for- 
ward in a dozen years, and in a single year five appeared. He 
attributed the positive charge of the sea breeze to the electrifica- 
cation of the air by the spray from the breaking waves; Lenard 
has shown that the spray of pure water gives a negative charge 
to the air, while that of salt water communicates a positive charge. 
He believed the dust of clouds to be acquired locally, except that 
at high altitudes, which we know to be carried long distances. 
A proof of this is to be found in the Himalayas, where certain val- 
leys are dusty and others fairly free from dust, although all receive 
the wind from the Indian plains. His observations of nacreous 
clouds in England had led him to connect them rather with the 
ice particles of cirrus clouds than with dust. To this latter point 
Prof. Michie Smith replied that the nacreous appearance fits the 
edge of the cumulus so closely that he believes the two to be 
connected.—WVature, lii, 534. 
