so remarkable a fog was still unanswered. Omitting this unascertained
primary cause, then, Professor Williams, of Harvard College, who
subsequently made a thorough investigation of the matter, gave it as
his opinion that this unprecedented quantity of vapor had gathered
in the air in layers so as to cut off the rays of light, by repeated
refraction, in a remarkable degree. He thought that the specific
gravity of this vapor must have been the same as that of the air,
which caused it to be held so long in suspension in the atmosphere. In
this case the extent of the darkness would coincide with the area of
the vapor, and it would continue until a change in the gravity of the
air caused the vapors to ascend or descend. In some places when the
darkness cleared it was as if the vapor was lifted and borne away by
the wind like a dark pall, and in others, after a period of intense
darkness the atmosphere gradually lightened again. In our day, a
phenomenon of this kind would be thoroughly investigated to its most
remote possible cause; but then owing to the sparse settlement of the
country and the difficulties of travel, the investigation of distant
causes could not be made. Large fires may have prevailed that spring
in the forests of Western New York and Pennsylvania--a region then an
absolute wilderness--the smoke of which was borne through the upper
regions of the atmosphere, to fall when it came to a locality of less
buoyant air, down to the lower strata. We say these fires may have
recently preceded this day, and served as its sufficient cause, but
we have only presumptive evidence that they did occur. Had Professor
Williams entertained a supposition of the previous existence of such
fires, he had then no means of verifying it, and long before the
advent of railroads and telegraphs, or even of stage lines, the
scientific theories of the dark day had passed from the general
memory.