THE MECHANICS OF VENTILATION. 471 



stance, where occupancy is at intervals, and then only for a short time, the 

 first will decide the choice of method. In buildings occupied more or less 

 continuously, the second consideration should have more weight, while in 

 ■dwelling houses the second and third together should influence the choice. 



The cheapest method of heating is bj^ stoves, more than ninety per cent, 

 of the heat being realized in this way. The first cost is also much less than 

 by the other methods. 



Comfortable and healthful heating is obtained b}^ open fire-places, two 

 types of which may be distinguished: 1 — Ordinary fire-places. 2 — Yenti- 

 lating fire-places. 



The ordinary fire-place takes its supplj' of air directly from the room and 

 heats solely by radiation. Its heating effect does not exceed .fourteen per 

 <3ent. of the total amount of heat produced. It is healthful and comfortable 

 but not at all economical. 



Captain Galton, of the British array, invented a form of the ventilating 

 fire-place which furnishes a very satisfactory solution of the problem of 

 pleasant, healthful heating, combined with excellent ventilation. It con- 

 sists of a grate with a flue leading therefrom as in the ordinary arrangement. 

 The flue, however, is of a good conducting and radiating material, and passes 

 up through an exterior flue, into which air is admitted from the outside at 

 the back of the grate. This air is warmed by coming in contact with the 

 interior flue, which gives it an upward tendency, and is withdrawn near the 

 ceiling into the room in which the fire-place is located, or it may be con- 

 ducted into the rooms of the second storj". The air to support combustion 

 is taken from the room warmed as in the previous plan. The heating effect 

 realized by this method is thirty-five per cent, of the total heating power 

 of the fuel. 



Heating by steam or hot water is pleasant, and when in connection with 

 proper ventilation exceedingly healthful. The heating effect realized is 

 nearly ninety per cent. 



Hot air, when properly regulated, is pleasant, and in connection with 

 ventilation, healthful. Heating effect seventy-five per cent. 



At a temperature of 60°, 13.09 cubic feet of air weigh exactly one pound. 

 A cubic foot at the same temperature weighs 534 grains. As already re- 

 marked, air in common with all other ponderable bodies obeys the laws of 

 gravitation, and, because of the slight attraction between its particles, it- is, 

 like gases genei-ally, extremely sensitive to changes of temperature or 

 pressure. -'^ -^ -'^ * * ''^ ^^ * 



According to the law of physics, known as the principle of Archimedes, 

 a body plunged into a fluid loses a part of its weight equal to the weight of fluid 

 displaced. This law may apply to three different cases : . ^ 



1. — The weight of the body may exceed the weight of the fluid displaced, 

 or in other words, the mean density of the body may be greater than that 

 of the fluid ; in this case the body sinks. 



