2G0 WELLS'S NATUKAL PHILOSOPHY. 



_ . As tho pressure of steam increases with its temperature, the 



pressure of pressure upon the interior of the boiler may also be known by 



steam be m- means of a thermometer inserted into tlie boiler. Thus it has 



dicatea by a 



thermometer? been ascertained that steam at 212° balances the atmosphere, 



or exerts a pressure of 15 pounds per square inch ; at 250°, 



30 pounds; at 275°, 45 pounds; at 294°, 60 pounds, and so on. 



„ ., ,, 585. The steam-whistle attached to locomotive and other 



Describe the . . , , , . , . » 



steam-whistle. engmes is produced by causmg the steam to issue from a 



. narrow circular slit, or aperture, cut in the rim of a metal cup; 



directly over this is suspended a bell, formed like the bell of a clock. The 



Bteam escaping from the narrow aperture, strikes upon the edge or rim of the 



bell, and thus produces an exceedingly sharp and piercing sound. The size 



of the concentric part whence the steam escapes, and the depth of the bell 



part, and their distance asunder, regulate tho tones of the whistle from a 



shrill treble to a deep bass. 



SECTION V. 



WARMING AND VENTILATION. 



Upon what ^86. lu tliG Warming and ventilation of 



thl°"warraing Ijuilcllngs, tttG cntiro process, whatever expe- 

 of'^'^buumngs tlients may be adopted, is dependent upon the 

 depend? cxpansion and contraction of air ; or in other 



words, upon the fact that air which has been heated and 

 expanded ascends, and air which has been deprived of 

 heat, or contracted, descends. 

 What is ven- ^^'^ • Ventilation is the act or operation of 



tiiation? causing air to j)ass through any place, for the 

 purpose of expelling impure air and dissipating noxious 

 vapors. 



The theoretical perfection of ventilation is to render it impossible for any 

 portion of air to be breathed twice in the same place. 



„,. , In the open air, ventilation is perfect, because the breath, as 



Wnere is ven- , , ■, . \ ,- , , 



tiiation perfect ? it leaves the body, IS warmer and lighter than the surround- 

 ing fresh air, and ascending, is immediately replaced by an 

 ingress of fresh air ready to be received by the next respiration. 

 .^ . . Common air consists of a mixture of two gases, oxygen an(J 



once respired nitrogen, in the proportion of one fifth oxygen to four fifths 



unwholesome ? 



nitrogen. By all the forms of respiration or breathing, and 

 of combustion, the quantity of oxygen in atmospheric air is diminished and 

 impaired, and to exactly the same extent is air rendered unwholesome and 

 unsuitable to supply the wants of the animal system. 



