58 DISEASES OF CATTLE, SHEEP, GOATS, ETC. 



to warm the incoming air by the consumption of a little more fuel. 

 That is the second item of expense. It remains for us, therefore, to 

 sit down and count the cost, whether we will put up with the dangers 

 and inconveniences and discomforts of bad ventilation, or pay the 

 charges for introducing and keeping up of a system of changing and 

 purifying the air that we breathe in our dwellings, or that the animals 

 under our charge breathe in their stables. 



What Is Ventilation? Theoretically, perfect ventilation consists 

 in removing the unwholesome products that are diffused through the 

 air of occupied rooms, as rapidly as these products are formed. But 

 perfect ventilation exists only in theory. Practically, we are satisfied 

 if we can remove a certain portion of these products, carbonic acid 

 gas and moisture from the lungs, and dilute the remainder to a 

 definite standard, by mixing with pure fresh air. An ordinary test 

 of a system of ventilation, is to determine the percentage of carbonic 

 acid gas present in the air of the room, and compare this with the 

 amount of the same constituent that is found in pure air. In any 

 case, ventilation consists in removing certain portions of the air, and 

 continually introducing fresh air. The rate at which this is done will 

 fix the degree of completeness of the ventilating arrangements. For 

 this constant removal and introduction of fresh air, certain forces 

 operate, which forces depend upon certain physical principles. It is 

 important that every man who is thinking of ventilating his house or 

 stables, should understand these principles, in order that he may 

 adapt the suggestions that are made in this pamphlet to suit his own 

 circumstances. 



The Problems in Ventilation. The object to be gained in venti- 

 lation is the maximum quantity of fresh air. But with the introduc- 

 tion into a room of large quantities of fresh and generally cold air, we 

 must guard against the possible consequences, viz., drafts, cold, and 

 dripping or condensation of moisture within the room. The prob- 

 lems then are : 



(1) To find a force that will keep the air in motion: removing 

 foul air and bringing in fresh. (2) To introduce and distribute the 

 air so as to avoid drafts. (3) To prevent the ventilated room from 

 becoming too cold. (4) To prevent condensation of moisture in the 

 room. 



Natural Aids to Ventilation. Under the first of the above men- 

 tioned heads, there are certain natural forces or tendencies in the 

 gases of which air is composed, that aid in ventilating a room or 

 stable. The first of these we shall consider under the head of Gravity. 



Gravity. When the air is warmed, it has a tendency to expand, 

 and will do so if allowed. The result of the expansion is that it be- 

 comes lighter, bulk for bulk, than the cooler air. The warm air, as a 

 consequence of its lighter weight, is displaced by the cooler air, and 

 driven upward, on the principle that heavy bodies sink when im- 

 mersed in fluids that are lighter than themselves. A stone when 

 thrown on the top of water sinks to the bottom on exactly the same 

 principle that cold air flows in underneath or falls down through 

 warm air and forces the latter upward. This sinking of cold air and 



