hence it is that the means which are provided for the preservation of health are actually the causes of destruction of life. "There are two causes which more evidently act upon the health of troops in the field than any other, namely, moisture exhaled direct from the surface of the earth in undue quantity, and emanations of a peculiar character arising from diseased action in the animal system in a mass of men crowded together. These are principal, and they are important. The noxious effects may be obviated, or rather the noxious cause will not be generated, under the following arrangement, namely, a carpet of painted canvas for the floor of the tent; a tent with a light roof, as defense against perpendicular rain or the rays of a vertical sun; and with side walls of moderate height, to be employed only against driving rains. To the first there can be no objection: it is useful, as preventing the exhalations of moisture from the surface of the earth; it is convenient, as always ready; and it is economical, as less expensive than straw. It requires to be fresh painted only once a year." The effect of crowding men together in close quarters, illy ventilated, was shown in the prisons of Hindostan, where at one time, when the English held sway, they had, on an average, 40,000 natives in confinement; and this unfortunate population was every year liberated by death in proportions varying from 4000 to 10,000. The annual average mortality by crowded and unventilated barracks in the English army has sometimes been enormous, as at Barrackpore, where it seldom fell far short of one tenth; 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.