366 HUMPHREYS: PRODUCTION OF CLIMATIC CHANGES 



Doubtless there have been several contributing causes of cli- 

 matic change, but it is the specific purpose of this paper to dis- 

 cuss only one of these, — a cause that during historic times has 

 often been fitfully operative, and concerning which we have 

 much definite information. 



The cause or factor in question is the presence of volcanic dust 

 in the upper atmosphere. 



Volcanic dust has often been mentioned as a possible cause 

 of cold seasons. Thus in May, 1784, Benjamin Franklin (and 

 he may not have been the first) wrote as follows: 



During several of the summer months of the year 1783, when the 

 effects of the sun's rays to heat the earth in these northern regions 

 should have been the greatest, there existed a constant fog over all 

 Europe, and great part of North America. This fog was of a perma- 

 nent nature; it was dry, and the rays of the sun seemed to have little 

 effect towards dissipating it, as they easily do a moist fog, arising from 

 water. They were indeed rendered so faint in passing through it, that, 

 when collected in the focus of a burnhig-glass, they would scarcely 

 kindle brown paper. Of course their summer effect in heating the earth 

 was exceedingly diminished. 



Hence the surface was early frozen. 



Hence the first snows remained on it unmelted, and received con- 

 tinual additions. 



Hence perhaps the winter of 1783^ was more severe than any that 

 happened for many years. 



The cause of this universal fog is not yet ascertained. Whether it 

 was adventitious to this earth, and merely a smoke proceeding from 

 the consumption by fire of some of those great burning balls or globes 

 which we happen to meet with in our course round the sun, and which 

 are sometimes seen to kindle and be destroyed in passing our atmos- 

 phere and whose smoke might be attracted and retained by our earth; 

 or whether it was the vast quantity of smoke, long continuing to issue 

 during the summer from Hecla, in Iceland, and that other volcano 

 which arose out of the sea near that island, which smoke might be 

 spread by various winds over the northern part of the world is yet 

 uncertain. 



It seems, however, worth the inquiry, whether other hard winters, 

 recorded in history, were preceded by similar permanent and widely 

 extended summer fogs. Because, if found to be so, men might from 

 such fogs conjecture the probability of a succeeding hard winter, and 

 of the damage to be expected by the breaking up of frozen rivers in 

 the Spring; and take such measures as are possible and practicable, 

 to secure themselves and effects from the mischiefs that attend the last.- 



2 See Sparks, "Life of Benjamin Franklin," 6: 455-457. (Cited in Proceedings 

 of the Amer. Phil. Soc, 45: 127. 1906.) 



