SCIENTIFIC SUMMARY. 
131 
as great accuracy may be obtained without the uncertain friction of a float, 
less liable to get out of order, and, consequently, more useful in the hands of 
the public. The principle is that of an independent, vertically-moving 
rod, registering instantly the point at which it comes into contact with the 
mercury of the barometer by thus forming an electric circuit. As an 
ordinary modification, he suggests that every hour the rod should be lowered 
through the whole range of the barometer by clockwork. On the formation 
of contact, an electro-magnet, by means of an armature, would cause a point 
carried along with the rod to be indented into the registering paper, or 
other material, on a revolving drum, similar to those already constructed 
for the purpose, to go for a week at a time. For more accurate purposes 
still the drum might be made larger, — say five inches in diameter, and 
fourteen inches long, and divided vertically into two inches for every day. 
In this case the drum would have to be moved upon its vertical axis every 
time, through a division equal to the interval of registration, — one-twelfth 
of an inch for one hour, for instance — and the recording-point would remain 
stationary, except when acted on by the magnet. At the hour, a simulta- 
neous motion would have to be given to the rod and the drum, — the first 
vertical and descending, the other horizontal and revolving — so that one 
inch and a half of the one would be equal to fifteen inches circumference of 
the other. The electric current, by means of the magnet as before, would, at 
the instant of contact, record the point at which this took place, and, con- 
sequently, the height of the barometer at that time. 
The Physics of Absorption . — The curious fact pointed out by Pouillet in 
1822, that when a fluid is absorbed by a porous substance, a rise in tem- 
perature occurs, has given origin to some strange explanations and discus- 
sions. The subject has recently been taken up by Jungk, who attributes, 
the alteration in temperature to the formation around each particle of the porous 
body of a thin layer of fluid, “ in which the individual molecules move with 
much less freedom ; thus pointing to a condensation of the fluid in those parts.” 
In support of his theory, he quotes a paper by Rose, on the errors which 
arise in the determination of the specific gravity, when the substance is 
weighed in a state of fine subdivision. The finer the particles of the body under 
examination, the greater will be the resulting specific gravity. He proceeds 
by assuming that the temperature of a body rises or falls when, by any 
external means, it is caused to assume the condition induced by the subtrac- 
tion or addition of heat respectively. Applying this in the case of water, 
it would follow that when it is absorbed by a porous substance, the tempera- 
ure should either rise or fall according as the water is below or above 
4° C. — the point of maximum density. This, in fact was found to be the 
case, and the results of his experiments may be shortly stated as follows : — - 
1. The temperature of water, when absorbed by sand, is raised or lowered, 
according as it was previously either above or below 4° C. 2. Water at 
0°, when absorbed by snow, is lowered in temperature. 3. The phenomenon 
may be regarded as a consequence of the condensation of the water on the 
surface of the absorbent body. — Yide Poggendorjfr ’s Annalen, No. 6, 1865. 
