394 
MATURE 
LAugust 27, 1885 
instead of a low one, as in the case of the V’s. The 
wedges seem to shoot up in front of cyclones and V de- 
pressions and to travel along before them. On the front, 
or east side, the weather is very bright, and the wind is 
north-west and moderate, while the temperature is that 
due to excessive radiation. On the rear, or west side, 
where the barometer begins to fall, the wind turns to 
south-west, and the sky overcasts in that peculiar manner 
which first gives a halo, and then gradually becomes black, 
without true cloud as the cyclone approaches. At the 
extreme north point of the wedge a shower or thunder- 
storm is sometimes observed. 
Straight Isobars.—In these the pressure is high on one 
side and low on the other, without any definite cyclone, 
the isobars running straight across the slope which joins 
the regions of high and low pressure. Straight isobars 
are never persistent, and the area which they have occu- 
pied is usually traversed by a cyclone of greater or less 
intensity. For forecasting purposes the indications are 
for cool, cloudy, unsettled weather, the wind from moderate 
to fresh, according to the gradients, to be followed soon 
by rain, as a cyclone forms or comes up. 
Cols—The col consists of a neck of low pressure 
between two anticyclones. The wind is always light and 
the weather quiet, but variable in appearance, owing to 
the local influence of radiation. Though the general 
position is sometimes nearly stationary, the weather is 
rather variable, owing to the tendency of the depression 
which lies on the north-west to develop a secondary in the 
col. Hence in forecasting, though it is possible to tell 
what the weather would be in the col at any moment, the 
future course of the weather is subject to much uncertainty. 
Mr. Abercromby devotes a considerable portion of his 
work to a discussion of weather-types and sequence. 
With reference to Western Europe, there are at least-four 
well-marked types of weather: 1, The Southerly, in which 
an anticyclone lies to the east or south-east of Great 
Britain, while cyclones coming in from the Atlantic either 
beat up against it or pass towards the north-east. 2. The 
Westerly, in which the tropical belt of anticyclones is 
found to the south of Great Britain, and the cyclones 
which are formed in the central Atlantic pass towards the 
east or north-east. 3. The Northerly, in which the 
Atlantic anticyclone stretches far to the west and north- 
west of Great Britain, roughly covering the Atlantic Ccean. 
In this case cyclones spring up on the north or east side, | 
and either work around the anticyclone to the south-east, 
or leave it and travel rapidly towards the east. 4. The 
Easterly, in which an apparently non-tropical anticyclone 
appears in the north-east of Europe, rarely extending 
beyond the coast-line, while the Atlantic anticyclone is 
occasionally totally absent from the Bay of Biscay. The 
cyclones then either come in from the Atlantic and pass 
south-east between the Scandinavian and Atlantic anti- 
cyclones, or else, their progress being impeded, they are 
arrested or deflected by the anticyclone in the north- 
east of Europe. Sometimes they are formed to the south 
of the north-east anticyclone, and advance slowly towards 
the east, or, in very rare instances, towards the west. 
Mr. Abercromby next explains the use of various aids 
to forecasting, and gives some detailed examples of suc- 
cessful and unsucessful forecasts. 
In concluding his work, Mr. Abercromby gives some 
remarks on forecasting generally, and points out that in 
many cases of small disturbances the minor features are 
so local that it is only the general character of the 
weather which can ever be forecast. Owing to the rapid 
nature of all meteorological changes, forecasts can never 
be issued very long in advance. The British forecaster 
labours under peculiar difficulties from his geographical 
position. Situated on the most outlying portion of Europe, 
and in the very track of storms which almost always 
advance from the westward, he has no intimation of an 
approaching cyclone till it is actually on him, Mr. 
Abercromby’s opinion is that, however carefully the relation 
of weather to isobars may be defined and the nature of their 
changes described, the judgment which experience alone 
can give to enable a warning to be issued must ever 
depend on the professional skill of the forecaster. 
RADIANT LIGHT AND HEAT" 
I. 
The Theory of Exchanges 
T was known at a comparatively early period that if a 
body be placed in an enclosure of constant and 
uniform temperature, it will ultimately attain the tempera- 
ture of this enclosure. 
To fix our ideas, let us suppose that we have a chamber 
surrounded on all sides by walls which are kept at the 
temperature of boiling water (100° C.), and Jet us further 
suppose for the sake of simplicity that there is no air in 
this chamber, so that no heat can be carried about by 
movable particles of gas. If under these circumstances 
we put a cold body into the chamber, it will ultimately 
reach 100°, at which temperature it will remain. This is 
a statement of the doctrine of temperature equilibrium ; 
but this equilibrium may be of two kinds—for it may 
either be a statical equilibrium, in virtue of which two 
bodies at the same temperature cease to radiate to each 
other, or it may be a dynamical equilibrium, in virtue of 
which each of these bodies independently radiates heat to 
its neighbour, receiving back, however, just as much 
heat as it gives out. In either case the ultimate result 
will be equality of temperature, and the only difference is 
with regard to the physical machinery by which this is 
brought about. In the theory of statical equilibrium the 
behaviour of two bodies of equal temperature with respect 
to heat may be compared to that of a man with respect to 
money who is getting neither richer nor poorer, because 
he is neither giving away nor receiving any money, 
whereas in the theory of dynamical or movable equi- 
librium the comparison is with the man who is getting 
neither richer nor poorer because he is receiving back just 
as much money as he is giving out. 
Now, we are all of us conversant with frequent 
examples of individuals of this latter class, but the condi- 
tion of things in this world is such that we cannot have 
any permanent example of the former, and similar con- 
siderations might convince us that if radiant light and 
heat be in reality a kind of energy, the theory of a n:oy- 
able or dynamical equilibrium must be much more cuit- 
able to such a constitution of things than that of a stat.cal 
or tensional equilibrium. Historically, however, the 
question of temperature equilibrium was not decided by 
considerations regarding energy, our conceptions of which 
were not then sufficiently advanced to be of much service 
to those who were engaged in the discussion. 
As the subject is one of great theoretical and practical 
importance, we shall proceed to give a short account of 
the circumstances attending the origin and development 
of what is now known familiarly as the theory of heat 
exchanges. About a century ago Prof. Pictet of Geneva 
made the following experiment :—He took two concaye 
metallic reflectors, and, reversing the ordinary mode of 
procedure, put ice or a freezing mixture in the focus of 
the one and a thermometer in that of the other, upon 
which the temperature of the thermometer was observed 
to fall. This effect would at once be explained if we 
could suppose that cold was a substantial entity capable 
of radiation and reflexion like heat. But it was im- 
mediately recognised that such an hypothesis is quite 
inadmissible, and Prof. Pierre Prevost, also of Geneva, 
was thus driven to propose for the explanation of this 
experiment the theory of a movable equilibrium of heat. 
It is very evident that such a theory will explain the 
* Continued from p. 327. 
i 
