412 MONTHLY WEATHER REVIEW. SEPTEMBER, 1907
which he pointed out the possibility of adischarge of negatively
electrified particles from the sun, which form nuclei of con-
densation, of which the earth receives its share on the side
turned toward the sun.
Maxwell, in his original presentation of the electromagnetic
theory of light, in 1873, pointed out that light should exert a
pressure, and showed how to calculate its amount. He found
that sunlight a t the earths surface should exert a pressure of
0.592 x 1O-lo grams on one square centimeter, a quantity so
small that i t would seem to be hopeless to observe it. Never-
theless Poynting thinks he has obtained experimental proof
of the existence of such a pressure. At the surface of the sun
a cubic centimeter of water, which on the earth weighs one
gram, would weigh 27A7 grams, and the pressure of sunlight
on a square centimeter would ainouut to 2.75 milligrams.
Such a mass would be attracted to the sun, but the case is
quite different with bodies of extreme minuteness. A cube of
water whose edge measures one-thousandth of a millimeter
[a micron or p ] mould experience an attraction of 37.17 x lo-
grams and the pressure of light on its side would be
2.75 x lOP3x 10Px= 27.5 x 10-l2, or rather more than its weight.
If, then, the cube has smaller dimensions than 1p it will be
repelled. The diameter of a molecule of hydrogen is possibly
about 0.0002 :I , and this contains perhaps W O O r corpuscles . Negatively electrified ions of this order of magnitude are given
off as kathode rays from the negative pole in a highly ex-
hausted tube, from substances upon which kathode rays iin-
pinge, from hot bodies, and from radioactive substances; and
these ions are capable of pro(1ucing fresh ions by collision
with the molecnles of the gas thru which they pass.
Professor Poynting remarks that  one effect of this radia-
tion pressure worthy of note woulcl be its sorting action on
dust particles. I f the particles in a dust cloucl circling round
the sun are of different sizes or densities, the radiation accele-
rations on them will differ. The larger particles will be less
affected than the sinaller, will travel faster round a gilen orbit,
and will draw more slowly in toward the sun. Thus a comet
of particles of mixed sizes will gradually be degraded froin a
compact cloud into a diffused trail lengthening and broaden-
ing, tlie finer dust on the inner and the coarser on the outer
edge .
A good account of Arrheniuss theory as applied to the ex-
planation of comets tails. the aurora, etc., has been given
in the Popular Science IlIonthly, Vol. LS, p. %is, New York,
1902, by Prof. John Cos, of RIc(4ill University.
With reference to the :turora, Professor Cos says:
Perhaps the most interesting application of Arrheniuss throry is hih
explanation of the aurora. In a well-hiown experiment the streaim of
negatik e particles forniing kathode rays in a Crookes tube are espo+ed
to a magnetic field, when they are seen to clescribe helices round the
lines of force. If the field is powerful enough they may tlius be bent
into a complete circle inside a Inoderately large tube.
Now the negative particles clibcharged from the sun arrive most thit.kly
over the equatorial regions of the earlh, which are most directly oppobecl
to him. Long before they reach any atmosphere dense enough to esciti.
luminescence, they are caught liy the lines of force of the eaithq niag-
netic field, which are horizontal orerthe equator, aucl have to follow them,
winding round them in heliceb whose radii are so much less than their
height above us t h a t tlie effect to a beholiler on the earth i5 as if they
moved along the lines of force. Over the equator there 1s little lumin-
escence for want of atmosphere. But as the lines of force travel riortli
and south they dill downward, making for the magnetic poles, over wliic~h
they stand vertical. Soon the particles find themsel%e+ in lower lager5
of the atmosphere, comparable in density with our highest artificial vacua,
and begin t o give out the darting and shifting lights of the kathode ray.
But this can Only be at the cost of absorption, and by the time the den5er layers of air are reached their energy is exhausted. Hence t h r dark
circles round the magnetic poles from M hich, as from behind a curtain,
the leaping pillars of the aurora rise. From this point of view it is sia-
niflcant that Dr. Adam Paulqen, who has made a +pecial study of the
Northern Lights, found so many point5 of correspondence between them
and kathode rays that in 1894 he was led tci regard the aurora ss a spe-
cial case of the latter, tho unable to give any account of their origin in
the upper atmosphere, such as is supplied by Arrheniuss theory.
I n communicating the above article Mr. W. M. Watts makes
the following appeal to American meteorologists :
I have come to the conclusion that i t is most desirable that other and
more accurate observations of the spectrum of the aurora bhould be ob-
tained. I would respectfully suggest that the duty of obtaining such
rests chiefly with you Americans, since your opportunities are, no doubt,
greater than ours. Simple arrangements seem to me quite sufficient.
In the flrst place choose a locality away from the lights of a town, prefer-
ably on an elevation. I should propose to construct a light observatory,
or dark room, of wooden framework, mounted so as to revolve horizontally
on a support, so as to oppose the slit bide to the brightest part of the
aurora. In one wall I would have a vertical slit, and outside of this the
largest lens I could olitsin for the purpose of concentrating the light upon
the slit. This lens need not be rtchromatic, and, therefore, need uot be
imtly. Insicle I would have ready the photographic apparatus, and
would supplement the photographit, attempts by eye observations of the
spectrum at every opportunity. For these eye observations I bhould
choose my binocular spectroscope with large gratings. For the photo-
graphs I should use a spectrograph, consibting of a Thorp (or Wallace)
gratiug as large as could be obtained, in front o f a lens (telescope) of
sufficient aperture to correspond to the grating. and of about 15 incheb
focal length: and last, but not least, I slioulcl use TYratten and Wain-
M right panchroniatic plates.
Measurements of wave lengths are easily macle with my binocular spec-
troscope. I have two different arrangements for he purpose, but in the
case of the aurora I should place a nitrogen (or other) vacuum tube so
as to occupy a portion of the slit-such vacuum tube being illuminated
at will by means of a switch a t the observers position.
Altho sucli great institutions as the Carnegie Observatory
on RIount Wilson, the Lick ()bservatory on Mount Hamilton,
the Ierlies Olservatory a t Williams Bay, the Harvarcl College
Observatory a t Calnbridge, and Mount Weather station at
Bluemoat, may have all the apparatus ancl men needed for
the above-mentioned research, yet the help of others favorably
located is still greatly to be desired. The outfit above recom-
mended by Mr. Watts is not expensive a8 compared with the
outtits for mauy other researches, but the work must be carried
out hy expert physicists such as are now found in every uni-
versity.-ErmoR.
~~
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SMITHSONIAN METEOROLOGICAL TABLES.
The steady clemand for the Smithsonian Meteorological
Tables has justified the preparation of a revised edition, dated
December, IBOG, which ie essentially a revision of the edition
of 1893, prepared by the late Mr. George E. Curtis, as repla-
cing the previous editions by Guyot, Libbey, and others.
The new eclition of 1906 contains an introcluction, followed
by tables that are classified as thermometric, barometric,
hygrometric, wind, geodetical, conversion, ancl miscellaneous.
Among the new or enlarged tables we note the following:
Marvins pressure of aqueous vapor a t low temperatures-
Eng.lish and metric measures.
Bigelows stanchrcl system of notation-foriuulas and con-
stants.
Hanzliks list of meteorological stations thruout the world,
with their latitudes, longitudes, and altitudes.
Ferxusson ani1 Claytons revision of international meteoro-
logical symbols, clouJ notation, and cloucl definitions.
The Beaufort notation for the use of navigators a t sea.
The acceleration of apparent gravity on the earth.
The total number of pages (Is, %O), as compared with that
in the revised eclition of February, 1896, Smithsonian Miscel-
laneous Collections, 1032 (lis, 274), shows that the changes
have been rery conservative. The voluiue as a whole is a
iuouument to the practical aspects of the work of the Smith-
soninn Institution.
KITE FLYING FROM MOUNTAIN TOPS.
I n his address, October 28, 1907, at the opening of the Aero-
nautical Congress in New Tork, N. T., the president, Prof.
Willis L. Moore, referred to the Weather Bureau kite work, as
Eollows: