Volume 2

A Textbook of the theory and practice of medicine ... / edited by William Pepper.

Date:
1893-94
Catalogue details

Licence: Public Domain Mark

Credit: A Textbook of the theory and practice of medicine ... / edited by William Pepper. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

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    beneath the stove, which is surrounded by a jacket of tin, or sheet jron, to force the air to ascend along the sides of the hot stove. This method is per- haps the best for country school-houses. In the direct method the heat is supplied by an open fire or by some kind of stove or radiator, and the fresh air is admitted at some other point or points and is not warmed. In this case the opening for the incoming air should be so placed and shaped that the current will be directed upward toward the ceil- ing. The simplest mode of doing this is by raising the lower sash of the win- dow four inches, closing the opening thus formed by a board, and allowing the air to come in through the crevice thus formed between the upper and lower sashes. In most houses the fresh-air supply comes in through crevices about the doors and windows, through the cellar from the soil into the halls and stairways, and directly through the walls of the house. Through the outer wall of an ordinary brick house, plastered but not painted or papered, about 7 cubic feet of air to each square yard pass per hour if the room is ten degrees warmer than the outer air. If the wall is damp, or is painted or papered on the inside, very little air can go through it. Stone walls are much less pervious to air than brick ones. In most dwelling-houses the outlets for foul air are the chimney-flues, and if the room have an open fireplace connected with such a flue, it is in most cases sufficient, even if there be no fire in the fireplace. In hospitals, schools, and other rooms where a number of persons are assembled, special outlet flues should be provided. The movement of the air through the room and in these outlet flues is generally produced by the difference in temperature between the air in the room and that of the outer air. Air expands 1 part in 491 of its volume for each degree Fahrenheit that it is heated, and, if it is free to expand, a cubic foot of warm air is therefore lighter than a cubic foot of cold air. The result of this is that the heavier cold air tends to flow in below and push up the lighter warm air. The pressure thus produced depends on the height of the column of warm air, being equal to this height multiplied by the difference in temperature between the warm and cold air divided by 491, and the velocity, if there is no friction, equals eight times the square root of this. Thus, if the height from the fireplace to the top of the chimney is 20 feet, and the difference in temperature between the air of the room and the outer air is 20° F., the theoretical velocity of the upward flow of the air in the chimney-flue would be ^X ~® , or a little over 7 feet per second. The actual velocity will 491 ' ^ J depend on the amount of friction at the points of entrance of the air into the room and in the outlet flue. If one fourth be deducted for this, the velocity in the above case would be 5.38 feet per second. If the flue were 1 foot square in cross-section, 5.38 cubic feet of air would pass through it every second. If the air in the flue and room is colder than the external air, it will fall instead of rising; the current will be reversed. It is not desirable to place upcast flues in outer walls, for the air in them is liable to be chilled and the