460 : 
NATURE 
| March 15, 1883 
heights. 
foreign to our graphic measures, the free roving aérilian set may 
all occupy a common place in this foreign scale of space, and 
that a massed and a moored aérilian point may have the same 
position in graphic space without impinging on each other, as 
such points are not at the time at one and the same place 
in the foreign space. The moored or bound ether may thus 
traverse the space occupied by the massed ether of gross 
matter without mutual interference ; but, whether superposed or 
not in ordinary space, the pair of ether sets which compensate 
resultant actions of two gross-clustered sets of a pair of points 
of baric matter, will form, however they may mingle graphic- 
ally, two orbs of ether exerting each (corporately) exactly 
counter-equal free-orb couples. 
If for example the baric points B B’ are urged towards each 
other with a ponderomotive force or flow of ordinary baric 
momentum F,, by the motor actions on them of two ether-orbs 
A, A,’ in counter equal intensities, force-momentum only will be 
transferred from A, to A,’ ; while the tractive momentum (or as 
we may presume, the heat-energy) accompanying it will only be 
transferred from A, to B, and a similar transfer of thermal energy 
or tractive-momentuim will at the same time take place from 
B' to Ay’. 
Should it, in the next place, be required to oppose the action F, 
by an equal counter-force Fy, a pair of ether-orbs A, A,’ must be 
superadded to those already urging B and By’, so as to urge them 
in the opposite direction. It will be seen from the figure that 
the total effect of this and of the previous orb-pair’s actions will 
simply be tbat the pair of ether-orbs connected with B will 
transmit motor energy from one to the other (from A, to Ay), and 
the other ether-pair will also transfer an equal amount of energy 
contrariwise from one orb to the other, without any leakage of 
ordinary momentum occurring at B and By’, by the neutralised 
action, into the channel B B’, 
Neweastle-on-Tyne, February 10 
(Zo be continued.) 
A. S. HERSCHEL 
Terrestrial Radiation and Prof. Tyndall’s Observations 
In Nature, vol. xxvii. p. 377, I see a notice on Prof. 
Tyndall’s observations on terrestrial radiation, with the author’s 
concluding remarks, that meteorologists should not be offended 
by his saying that from outsiders equipped with the necessary 
physical knowledge they may expect valuable aid towards intro- 
ducing order and causality among their observations. May I be 
permitted to state that Prof. Tyndall will give no offence, at 
least to the meteorologists whose works are advancing this 
science at the present time. 
Prof. Tyndall tries to prove by his observations the extreme 
importance of vapour of water as a check to terrestrial radia- 
tion, and he mentions the much greater difference between a 
thermometer in the air four feet from the ground and another on 
cotton wool on a morning when snow was lying on the ground 
than on other nights, equally clear, but with higher tempera- 
tures of the air and no snow. Now it is well known that, parz 
passu, a surface of snow will be colder than a surface without, 
because (1) snow is an excellent radiator ; (2) because, as a very 
bad conductor, it shelters the surface from the influence of the 
higher temperature of the soil. In the observation on December 
10, the thermometer on cotton wool was so coli because it was 
under the influence of the cold radiated by the snow, and besides 
immersed, so to say, in the coldest stratum of air near the 
ground. Tomy mind, the manner in which the observation was 
conducted does not prove what Prof. Tyndall advances. To 
isolate, so to say, the influence on radiation of the atmosphere 
itself, he should have placed, between two poles, at some feet 
above the ground, a plank, and on it his cotton wool and ther- 
mometer. No doubt that this thermometer, isolated from the 
snow, should have shown a higher temperature than his thermo- 
meter placed on the surface on cotton wool. 
It is thus quite conceivable that, in a scale of space | 
Prof. Tyndall lays great stress on the fact that the difference 
between the temperature in the air and on the ground was less in 
clear nights with a higher temperature and greater quantity of 
vapour of water in the air, and sees in this a confirmation of his 
opinion on the great influence of vapour of water in checking 
radiation. I do also see in this the influence of vapour of water, 
but not of its absolute quantity, but of relative humidity. Once 
the dew-point is attained, the cooling of the thermometer on the 
ground is arrested. The whole question between Prof. Tyndall 
and many physicists and meteorologists is this: nobody negates 
the influence of vapour of water on terrestrial radiation, but Prof. 
Tyndall ascribes this influence to vapour in the gaseous state, 
while his opponents hold the opinion that in this state vapour of 
water has a diathermacy scarcely different from dry air, while, 
condensed in small ice crystals or water droplets, it really inter- 
poses a very efficient screen to terrestrial radiation, even if, which 
sometimes is the case, it is perfectly transparent to light, ze. 
invisible to our eye. Another influence of water on terrestrial 
radiation is admiited by all: that is, that of the latent heat in the 
deposition of dew and hoar frost. 
If we wish to make meteorological observations bearing on 
the question, the following modus oferandi should be adopted : 
(1) observations should be made in climates where, with a ten- 
sion of vapour greater than that which obtains in England in 
winter, the relative humidity is yet so small that there is no dew 
on clear nights, or at least it appears rather late ; (2) three ther- 
mometers placed on cotton wool, but at different heights above 
the ground should be observed, say one on the ground, and the 
others at heights, say from 10-100 feet above. 
If Prof. Tyndall’s views are right, the highest of the ther- 
mometers should show by far the lowest temperature, as it 
is not screened from radiation by the vapour of water dif- 
fused in the lowest stratum of air. I think every meteoro- 
logist will express the opinion that there will be scarcely 
a difference in this case. As to the observations in dif- 
ferent climates, those made where the relative humidity is low 
should .ive no greater difference between the thermometer in 
the air and on the wool than the observations in England on 
clear nights, with the same vapour tension, if Prof. ‘lyndall’s 
hypothesis be admi‘ted. Ithink we have already many observa- 
tions which prove that, with vapour-tensions much above o”"181 
(or 46mm.), ze. above that of saturation at 32° F., terrestrial 
radiation is very great, if only the sky is clear and the relative 
humidity small. No doubt the decrease of the temperature of 
the air from the midday maximum to the night minimum is 
caused by terrestrial radiation. I give some figures from the 
observations at Biskra, in the Algerian Sahara,! 
Difference of 
AE Mean Tension of Relative Amount 
By nae temperature. vapour. humidity. of cloud. 
January 25°4 56°8 0°264 61 1°6 
August 39°2 89°6 0°557 40 o8 
October ... 35°6 68°4. 0°432 58 o'9 
In an arid climate in low latitude the non-periodic variations 
are but small, and the difference between the maxima and minima 
is very near to the daily range of temperature. As the amount 
of cloud is very small in all three months taken here, the con- 
ditions for terrestrial radiation are very favourable. If vapour 
of water in the gaseous state impeded terrestrial radiation so 
much as stated by Prof. Tyndall, we should expect to find the 
daily range smaller in August than in January, on account of the 
double amount of vapour in the air. The reverse is the case, 
the daily range being by 14°8 greater in August thanin January. 
Has anybody observed a daily range of 39°2 in England, be the 
amount of cloud and the vapour-tension ever so small ? 
I must add that in all observations bearing on terrestrial radia- 
tion we must not forget that other substances besides water in its 
three states may interpose a screen to radiation. I mean espe- 
cially dust and smoke of all kinds, Now far from large cities, 
there are many reasons why in winter, especially when the 
ground is covered with snow, the air will hold less of these im- 
purities than in summer, as in winter there are no fires of forests 
and peat-bogs, there is little inorganic dust, because the humidity 
of the soil, and still more so the snow, prevent it; organic dust, 
germines, &c., are also absent, or present in very small quaati- 
ties, on account of the small amount of plant and lower animal 
life. The absence of dust and smoke explains the great purity 
of the air in winter, so favourable to solar and terrestrial radia- 
t « Annales du Bureau Central Météorologique de France,” 1879, vol. i. 
