MxzzsoN.—On the recent Sun-glows. 888 
of any kind settles from the air on a surface warmer than itself very 
reluctantly. Once in the higher air, therefore, dust would be perhaps able 
for a long time to resist gravitation. Professor Trowbridge believes that the 
upper currents of air would tend—by their motion, I presume—to keep the 
dust in suspense. Of these said upper currents our knowledge, as Lockyer 
remarks, is really very limited ; but the investigation of these sun-glows 
` and a careful comparison of the dates of their appearance in different 
places may perhaps extend our knowledge in this respect as well as in 
others. And this brings up 
8. The actual dispersion or diffusion of the dust as a great difficulty. As 
Hazen says, the currents of the upper air, if they caused the diffusion, must 
have acted in opposite directions, whereas we have always understood that 
the upper air moves steadily in one current from west to east. In answer 
to this I can only repeat what has been said. Nothing is certainly known 
about the higher aerial eurrents. And as I have just now ventured to 
suggest incidentally, the electrical repugnance of the particles of dust to one 
another may have had something to do with their diffusion. 
4. Hazen regards the rapidity of the dust dispersion as a stumbling 
block. The upper currents or something else must have carried the dust 
12,000 miles (to Barinas, Venezuela) in 150 hours, i.e., at the rate of 80 
miles an hour; whereas observations on Pike's Peak, 14,134 feet high, show 
the current there to be running at the uniform rate of 20 miles an hour. 
To this the reply at once occurs: an observation at the height of, say three 
miles, is not conclusive as to the rate of the wind throughout the upper 
regions, and we do not know at what height the line of dust was spread out. 
Moreover, Symonds thinks that the rapidity of the westerly dispersion may 
perhaps be accounted for by the rotation of the earth from west to east, so 
that the dust was as it were left behind by a process analogous to that which 
causes the trade winds in the tropics. But this would, it appears, only 
account for a progress of 440 miles a day, whereas that to Barinas was at 
the rate of 1,700 miles a day. Then, again, Symonds suggests that perhaps 
the dust got quite out of the earth’s atmosphere, and so was left a whole 
hemisphere behind, which idea Cowper Ranyard will not entertain at all, 
and I confess that I do not understand it. That the dispersion of the 
unusual material in the upper air (for the difficulty is the same whether 
dust was the material or not) was exceedingly rapid in some directions, 
particularly to the west, is certain. But, perhaps, the earth’s rotation, the 
upper-air currents, and the electrical repulsion combined, were sufficient 
propelling causes. It is, you will notice, in connection with this branch of 
the subject that accurate observations of the dates of the first appearances 
