58 Marshes and their Effects on Human Health. 
and the number diminished yearly, as nature removed the cause 
by giving frosts and rains to stop the progress of the great body 
of vegetable decomposition, which had charged the atmosphere 
with its poisonous qualities. 
We have already remarked that the surface of the town, as 
seen from the surrounding hills, pretty nearly marked the form of 
a basin. The curious will enquire then, why the fever did not 
extend all over the place. The reason is conclusive. The fogs, 
after they had risen, passed down the rail road track to the lower 
swamp which was broken up the deepest, and from this it passed 
up the lowland in a westerly and north-westerly direction, until 
its progress was arrested by the west hills, where it remained 
stationary until its fatal qualities were discharged in quantities 
sufficient to produce the effects already mentioned. 
The cases east of the rail road were few, a circumstance which 
may conclusively be attributed to the circumstance that these 
deadly fogs seldom passed off in that direction. Two causes for 
this result. First. The streams from the east mountains, the most, 
indeed with one or two exceptions all of them, pass off without 
any communications with these swamps, consequently there are 
no such direct channels for the ascent of vapor in that direction 
as on the west. The region on the east is more elevated of the 
two, which likewise operated as a barrier to its approach if natu- 
ral causes have effect, which we leave to others to decide. Second. 
The seasons were attended with frequent easterly winds, which 
of course, drove the vapor in a westerly direction. But it was 
not always in the presence of the east winds, that the fogs were 
most dense or longest in continuance. On the contrary, in the 
still atmosphere, when the vapor passed off, as in natural chan- 
nels, until its progress was arrested by the mountains, against 
whose sides it hung like a heavy cloud until the sunshine and its 
own exhausting power dispelled it. 
We have been particular in the foregoing relation of the cause 
and effect of a miasmatic atmosphere, arising from impure ex- 
halations from lowlands, because the case was so strongly marked, 
that it would appear that no mistake could be made in the matter. 
