550 PROFESSOE -T. H. P05^XTIX(4 ON RADIATION IN THE SOLAR SYSTEM : 
C/ 27 rT is a superior limit to the number of revohitions, even if we suppose the wav 
clear right up to tlie centre. 
Putting the numerical values we get 
C/27rT = Gl7’h(. 
Suppose, for example, that r = h — 493 X 3 X ; a = 1, then 
r/r = - 3-5 X lO^h 
If v'e multiply hv 3‘15 X 10', the seconds in a year, (r, r) X 3'15 X 10'= I'l X 10 
This implies that a sphere 1 ceiitim. radius and density 5‘5, starting with the 
velocity of the earth, and at its distance from the sun, will move inwards 
^ — of its distance in ahout 10,000 years. It cannot in all make so many as 
10.000 
GI X = 2‘35 X 10® revolutions. 
If we put « = O'OOl centim., since the effects are inversely as a, then its distance 
will decrease by about 1 in 10,000 in 10 years, and it cannot make in all so many 
as 2'3 5 X 10“ revolutions. 
If instead of starting fi-oni the distance of the earth, the particle starts from, 
say, O'l the distance, the effect in the radius is 100 times as great and the number 
of revolutions is \/l0 tunes less. Tlien with radius 1 centim. the distance decreases 
l)y -1— ill 100 years, and there are not so many as 80,000 reyolutions, while 
witli radius O'OOl centim. the distance decreases liy year, and there are 
not so many as 80 reyolutions. 
Small particles, therefore, eyen of the ordei' of 1 centim. radius, would be drawn 
into the sun, eyen from the distance of the eartli, in times not large compared Avith 
geological times, and dust jiarticles if large enough to absorb solar radiation would be 
swept in in a time almost comparable with historical times. Near the sun the effects 
are yastly greater. The application to meteoric dust in the system is obvious. 
Tliere should be a similar effect Avith dust and small particles circulating round the 
earth. If, for examjile, any of the Krakatoa dust Avas liloAvn out so far beyond the 
appreciable atmosphere, and Avas giA^ensiich motion tliat the particles became satellites 
to the earth, at no long time tlie du.st Avill return. A ring ot dust })articles moAung 
round a ])lanet and I’eceiA'ing heat either trom the sun or from the planet aa'III 
tend to draAV in to the planet. 
[Note added October 31.— Since the foregoing ...paper was printed I haye re¬ 
examined the theory of the pressure on a fully radiating surface Avhen in motion, and 
haye come to the conclusion tliat the change in pressui'e due to the motion is only 
half as great as that olitained on j). 545. In that iiiA^estigation the pressure Avas 
assumed to be ec^ual to tlie energy density, Avhether the surtace Avas at rest oi' m 
