Meeson. — Oil the recent Sun-glows. 383 



of any kind settles from the air on a surface warmer than itself very 

 reluctantly. Once in the higher air, therefore, dust would he 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 



3. 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 currents. 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 

 perbaps 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 Banyard 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 



