nth; that is to say, its garrisons were every year
decimated by fever or cholera, while the officers and other
inhabitants, who lived in well-ventilated houses, did not find the
place particularly unhealthy.

The same fact of general exemption among the officers, and complete
exemption among their wives, was observed in the marching regiments,
which lost by cholera from one tenth to one sixth of the enlisted men,
who were packed together at night ten and twelve in a tent, with the
thermometer at 96 deg.. The dimensions of the celebrated Black Hole of
Calcutta--where in 1756, 123 prisoners out of 140 died by carbonic acid
in one night--was but eighteen feet square, and with but two small
windows. Most of the twenty-three who survived until morning were
seized with putrid fever and died very soon afterward.

On the 1st of December, 1848, 150 deck passengers of the steamer
Londonderry were ordered below by the captain and the hatches closed
upon them: seventy were found dead the next morning.

The streams which intersect our great prairies have but a very sparse
growth of wood or vegetation upon their banks, so that one of the
fundamental causes for the generation of noxious malaria does not, to
any great extent, exist here, and I believe that persons may encamp
with impunity directly upon their banks.

PICKET GUARDS.

When a party is sufficiently strong, a picket guard should be stationed
during the night some two or three hundred yards in advance of the
point which is most open to assault, and on low ground, so that an
enemy approaching over the surrounding higher country can be seen
against the sky, while the sentinel himself is screened from
observation. These sentinels should not be allowed to keep fires,
unless they are so placed that they can not be seen from a distance.

During the day the pickets should be posted on the summits of the
highest eminences in the vicinity of camp, with instructions to keep a
vigilant lookout in all directions; and, if not within hailing
distance, they should be instructed to give some well-understood
telegraphic signals to inform those in camp when there is danger. For
example, should Indians be discovered approaching at a great distance,
they may raise their caps upon the muzzles of their pieces, and at the
same time walk around in a circle; while, if the Indians are near and
moving rapidly, the sentinel may swing his cap and run around rapidly
in a circle. To indicate the d