66 
NATURE 
ae , 
| Woo. 28, 1872 4 
necessary to a determination of the mean position is 
still more scarce, and still more data must be ob- 
tained to form a mean value, although the pack-ice 
girdle thus obtained can only have an ideal importance. 
If we follow the boundaries of this pack-ice girdle, we 
notice to the south of New Zealand a bend towards the 
south analogous to that of the drift-ice, only shifted in 
accordance with the general direction of the current ; also 
on the west side of Graham Land this tendency of the ice 
to bend towards the south is noticed, for it is always in- 
fluenced by the direction of the land. To the south of 
Kerguelen Land the pack will only be to the west of 
Termination Land, and, indeed, stretching in a direction 
which makes us suspect another bend towards the south, 
We must yet mention a sort of product of the drift, of 
considerable importance in ascertaining the condition of 
the drift and the ocean-currents, viz, the seaweed. 
This is loosened from its position especially by drifting 
icebergs, and turned adrift to the play of the waves. 
This conglomeration of plants, which reaches a littie 
above the surface, though the long fibres stretch deep 
down into the water, shows the currents very correctly, 
for it is as little influenced by the winds as a drifting 
bottle. The curve that limits the place of growth is con- 
fined, in the Atlantic and Indian Oceans, to about the 
6oth meridian of E. long., near to that of the drift-ice. 
In some places this also holds good for regions to the 
south of Australia and the south-east of New Zealand. 
At all events, it can thus be understood how those 
places which are free from drift-ice are also free from 
drift-weed, a point of some importance when we consider 
the fact that drift-weed is the attendant of icebergs. 
According to the Dutch voyagers, on the west of the 
meridian of Paul and Amsterdam are found weeds dif- 
ferent from those on the east thereof, which points toa 
different current. 
(To be continued.) 
TYNDALL’S RESEARCHES ON RADIANT 
HEAT 
Contributions to Molecular Physics in the Domain of 
Radiant Heat. By John Tyndall, LL.D., F.RS. 
(Longmans, 1872.) 
pent’ years ago our knowledge concerning the ra- 
diation and absorption of heat was very meagre, 
We believe that the earliest experiment in this direction 
is to be found in that wonderful book containing the 
“Essayes” of the Florentine Academicians. There we 
meet with the fact that the heat of the sun, converged by 
a mirror, can ignite a pastile placed within a Torricellian 
vacuum. A little later, in 1682, Mariotte communicated 
to the Paris Academy of Sciences that the heat of a com- 
mon fire, made very sensible in the focus of a burning 
mirror, was entirely cut off by the interposition of a sheet 
of glass. The mirror in this case must have been of 
polished metal, These experiments were subsequently 
repeated and extended by Lambert, who, assigning the 
true cause, pointed out the necessity of employing metal, 
and not glass mirrors, in the reflection of heat from terres- 
trial sources. Lambert further showed that if the radia- 
tion from a clear fire were converged by a large glass lens, 
no heat was felt where! the brilliant focus was seen, 
| stance. 
whilst Hoffmann first collected the obscure heat of a 
stove to a focus by a metal mirror. : 
About a century ago Franklin made his experiments 
with bits of coloured cloth on snow, and found that, 
whereas colour strongly influenced the amount of solar 
heat absorbed, it made little or no difference on the emis- 
sion from a lamp or candle. Beyond the foregoing in- 
formation, we believe little more was known in this 
subject till, in 1777, Scheele published his famous treatise 
on “Air and Fire.” But a great deal of ignorant talk, 
clothing itself beneath a barbarous jargon, was prevalent — 
at that time. Putting much of this aside, Scheele asks 
himself the question, which he was unaware Lambert 
had already answered, whether it was really the heat, or 
only the light of a fire, that was reflected froma metal 
mirror ; but first he seeks to know whether there are dif- 
ferent kinds of heat. “There will be many,” he remarks, 
“who will not hesitate to give an answer to my queries ; 
for I am well acquainted with the vague phrases accor d- 
ing to which everything is called Fire that hath a distant 
similarity to it. But I am of opinion,” he naively adds, — 
“itis best not to begin to read before one knows the 
elements of the alphabet, and to withhold an answer till — 
one has reflected on the following experiments.” Then 
comes a series of admirably simple experiments, pregnant 
with important results. As they deserve to be more 
widely known than they seem to be, the reader will par- 
don our quoting Scheele’s own words at length. They 
are to be found in section 57 of his work already alluded 
to :—‘ From these experiments ”—made in winter, before 
an open stove, and with only a candle, a concave mirror, 
and sheet of glass and metal, for apparatus—“it ap- 
pears,” says Scheele, “that the heat, mounting with 
the air in the stove, and passing through the chim- 
ney, is materially different from the heat passing 
through the door of the stove into the room, That 
the latter heat departs from the centre, where it is 
generated in straight lines, and is reflected from 
polished metals under an angle which is equal to that of 
incidence. That it unites not with air, nor can be 
diverted by a current of air, into a direction different 
from that which it originally had received. For that very 
reason, the vapours of the breath are visible in this great 
heat ; for since air and heatare really united during sum- 
mer, and warm air can dissolve more water than cold air, 
it likewise hence appears that this kind of heat is not 
united with air, nor has this kind of heat probably rare- 
fied the air ; and of consequence it becomes evident why 
it causes no tremulous motion in sunshine. You may, by 
means of a glass mirror, separate the light from this 
heat, when the heat remains in the glass, and the radiant 
light yields no heat. Hence it follows that the heat, 
passing through the door of the stove, coincides, in some 
points, with the light, but is not yet quite become light, 
since it is not reflected in the same manner from a glass 
surface as from a metallic one—a remarkable circum- 
This heat is soon changed into the usual heat, 
whenever it unites with a body, which may be observed 
in the glass, andin the metallic concave mirror blackened 
by soot, and in more instances. Represent to yourself,” 
he concludes, “a little hillock of burning coals ; in this 
case the heat darting from this hillock all around is that 
which may be reflected by a metallic polished plate ; that, 
