250 
NATURE 
[Fuly 15, 1886 
represents a combination of said device and a pyrheliometer 
differing materially from Pouillet’s instrument, by showing the 
true intensity of the ‘‘ fire” in the sun’s rays. 
The illustration presents a top view and a vertical section of 
the new instrument through the centre line. The upper part, 
composed of bronze, is cylindrical with a flat top, the bottom 
being semispherical, composed of ordinary glass. The top of the 
cylindrical chamber is provided with three circular perforations 
covered by a thin crystal carefully ground and polished. A 
thermometer having a spherical bulb is introduced through the 
side of the chamber, the bulb being central to the transparent 
semispherical bottom. A short parabolic reflector, shown in 
section on the illustration, surrounds the instrument, adjusted so 
that its focus coincides with the centre of the bulb of the thermo- 
meter. The compound cylindrical and spherical chamber is 
inclosed ina vessel containing water, appropriate openings at 
top and bottom being provided for maintaining constant circu- 
lation during experiments. Efficient means are also provided 
for exhausting the air from the internal chamber. The instru- 
ment is secured to the top ofa substantial table which, during 
experiments, faces the sun at right angles by the intervention of 
a parallactic mechanism. Movable shades are applied, by 
means of which the sun’s rays may be quickly cut off from, or 
admitted to, the parabolic reflector; while other shades enable 
the operator to admit or exclude the solar rays from the circular 
perforations at the top of the exhausted chamber. It will be 
readily understood that the parallel lines within the exhausted 
chamber, shown on the illustration, indicate the course of the 
solar rays passing through the crystal and the perforations at the 
top, while the converging radial lines indicate the rays reflected 
by the parabolic reflector. The »pper hemisphere of the thermo- 
metric bulb, it will be seen, receives the radiant energy of the 
sun’s rays which pass through the large central perforation ; 
while the lower half of the bulb will be acted upon by the rays 
passing through the small perforations. ‘These rays are reflected 
upwards by two inclined circular mirrors attached to the bottom 
of the exhausted chamber. It should be particularly observed 
that the areas of these inclined mirrors /ogetier should exceed 
the area of the great circle of the bulb of the thermometer suffi- 
ciently to make good the loss of radiant energy caused by the im- 
perfect reflection of the said mirrors, and also to make good the 
loss attending the passage of the solar rays through the crystal. 
A capacious water cistern, connected by flexible tubes with the 
external casing of the pyrheliometer, enables the operator to 
maintain the exhausted chamber at any desirable temperature. 
Engineers of great experience in the application of heat for the 
production of motive power and other purposes deny that the 
temperature of a body can be increased by the application of 
heat of a lower degree than that of the body whose temperature 
we desire to augment. The soundness of their reasoning is 
apparently incontrovertible, yet the temperature of the mercury 
in the instrument just described raised to 600° F, by means of 
the parabolic reflector, increases at onee when solar heat is 
admitted through the circular apertures, althouzh the sun’s 
radiant intensity at the time may not reach one-tenth of the 
stated temperature. It should be mentioned that the trial of 
this new pyrheliometer has not been concluded, owing to very 
unfavourable atmospheric conditions since its completion. For 
our present purpose the great fact established by the illustrated 
instrument is sufficient, namely that the previous temperature of 
a body exposed to the sun’s radiant heat is immaterial. The aug: 
mentation of temperature resulting from exposure to the sun, the 
pyrheliometer shows, depends upon the intensity of the sun’s rays, 
Regarding the temperature prevailing during the lunar night, 
its exact degree is not of vital importance in establishing the 
glacial hypothesis, since the periodical increment of temperature 
produced by solar radiation is only a fraction of the permanent 
loss attending the continuous radiation against space resulting 
from the absence of a lunar atmosphere ; besides, all physicists 
admit that it is extremely low. Sir John Herschel says of the 
night temperature of the moon that it is ‘the keenest severity 
of frost, far exceeding that of our Polar winters.” Proctor says: 
“A cold far exceeding the intensest ever produced in terrestrial 
experiments must exist over the whole of the unilluminated 
hemisphere.” The author of “ Outlines of Astronomy” has 
also shown that the temperature of space, against which the 
moon at all times radiates, is — 151° C, (—239°°8 F.), Pouillet’s 
estimate being — 142° C. (—223°6 F.). Adopting the latter 
degree, and allowing 81°*11 for the sun’s radiant heat, we esta- 
blish the fact that the temperature of the lunar surface presented 
to the sun will be 223°°6 less S1°"1, or - 142°°5 F., when the 
earth is in aphelion. — It will be well to bear in mind that when 
the earth is in the said position, the sun’s rays acting on the 
moon subtend an angle of 31’ 32”, hence the loss of heat by 
radiation against space will be diminished only o’oo0021 during 
sunshine. Nor should Herschel’s investigation be lost sight of, 
showing that stellar heat bears the same proportion to solar heat 
as stellar light to solar light. Stellar heat being thus practically in- 
appreciable, the temperature produced by stellar radiation cannot 
be far from absolute zero—an assumption in harmony with the 
views of those who have studied the subject of stellar radiation, 
and consequently regard Pouillet’s and Herschel’s estimate of the 
temperature of space as being much too high. 
Having disposed of the question of temperature, let us return 
to the practical consideration of the glacial hypothesis. The 
formation of annular glaciers by the joint agency of water and 
the internal heat of a planetary body devoid of an atmosphere 
and subjected to extreme cold is readily explained on physical 
principles. Suppose a sheet of water, or pond, on the moon’s 
surface, covering the same area as the plateau of Tycho, viz. 
50 miles diameter and 1960 square miles. Suppose, also, that 
the internal heat of the moon is capable of maintaining 2 
moderate steam pressure, say 2 lbs, to the square inch, at the 
surface of the water inthe pond. The attraction of the lunar mass 
being only one sixth of terrestrial attraction, while the moon’s 
surface is freed from any atmospheric pressure, it will be evident 
that under the foregoing conditions a very powerful ebullition 
and rapid evaporation will take place, and that a dense column 
of vapour will rise to a considerable height above the boiling 
water. It will also be evident that the expansive force within 
this column at the surface of the water will be so powerful at 
the stated pressure that the vapour will be forced beyond the 
confines of the pond in all directions with great velocity. No 
vertical current, it should be understood, will be produced, since 
the altitude of the column, after having adjusted itself to the 
pressure corresponding with the surface temperature of the water, 
remains stationary, excepting the movement consequent on con- 
densation from above. The particles of vapour forced beyond 
the confines of the pond, on being exposed to the surrounding 
cold, caused by unobstructed radiation against space, will of 
course crystallise rapidly, and in the form of snow fall in equal 
quantity round the pond, and thereby build up an annular gla- 
cier. As the radius of the vaporous column exceeds 25 miles, 
it will be perceived that, notwithstanding the rapid outward — 
movement, before referred to, some of the snow formed by the 
vapours rising from the boiling pond will fall into the same, to 
be melted and re-evaporated, 
In connection with the foregoing explanation of the formation 
of annular glaciers, their exact circular form demands special 
consideration. An examination of Ruthezford’s large photo- 
graph of the lunar surface shows that, apart from the circular 
form of the walls, the bottoms of the depressions are in 
numerous cases smooth, rising slightly towards the centre 
uniformly all round. The precision observable proves clearly 
the action of formative power of great magnitude. Referring to 
what has already been explained regarding the vaporous column 
of 25 miles radius, calculation shows that a surface temperature 
exerting the moderate pressure of 2 Ibs. to the square inch 
will produce an amount of mechanical energy almost incal- 
culable. Practical engineers are aware that the steam rising 
from a surface of water 10 square feet, heated by a very slow 
fire, is capable of producing an energy of 1 horse-power ; conse- 
quently a single square mile of the boiling pond will develop 
2,780,000 horse-power. This prodigious energy will obviously 
be exerted horizontally, as the weight of the superincumbent 
column of vapour balances its exfansive force precisely as the 
weight of our atmosphere balances its expansive force. But 
unlike the earth’s atmosphere, which is restrained from hori- 
zontal movement by its continuance round the globe, the vapour 
of the column of 50 miles diameter is free to move beyond the 
confines of the pond. A very powerful horizontal motion, 
especially of the lower part of the vaporous mass, will thus be 
promoted, acting in radial lines from the centre, the principal 
resistance encountered being the friction against the water. 
Considering that the friction against the surface of the ocean, 
caused by the gentle trade-wind, is sufficient to produce the 
Gulf Stream, we need no figures to show the effect on the water 
in the boiling pond produced by the vaporous mass propelled by 
an energy of 2 Ibs. to the square inch, in radial lines towards its” 
confines. A circular tidal wave of extraordinary power, together 
with a return under-current towards the centre, will obviously be 
the result. Butagreeably to the laws supposed to govern vortex 
ae WM * 
Pal he ee 
