HEATING THE RANGE 105 



of heat with a lesser consumption of coal, will maintain 

 temperatures during the night and toward morning, far 

 better than a small pipe system can. 



" It has been stated that the susceptibility of green- 

 houses to cooling winds makes them hard to heat. The 

 actual loss of heat through laps between the glass is next 

 to impossible to calculate. In the heating of dwelling 

 houses, hospitals, offices and all such buildings, engineers 

 calculate very closely the number of air changes per hour 

 and make proper provision for the same. In greenhouses 

 the number of changes of air per hour, whatever it might 

 be, is dependent first on the wind velocity or pressure, 

 on the roof pitch, on the shape of the roof, on the amount 

 of lap of the glass and on the size of the openings between 

 the laps of glass. The size of these openings, in turn, 

 depends upon the width of the glass used. The wider 

 the glass used, the greater the ' sag/ that is, the greater 

 the openings between the lights. Four widths of glass 

 are used, twenty-four, sixteen, fourteen and twelve inch, 

 although sixteen-inch may be taken as a standard, as it is 

 now being used almost exclusively in the best houses. 

 Fourteen and twelve-inch glass are seldom used. 



" It has been explained that the number of changes of 

 air per hour are hard to determine. Nevertheless, the 

 heat loss due to this cause is high, and for this reason it is 

 comparatively easier to heat a large house than it is to 

 heat a small house, or in other words, less radiating sur- 

 face is required per square foot of glass for a large house 

 than is required per square foot of glass for a small house. 

 The proportion of glass to cubic contents is not true for 

 all sizes and shapes of- houses ; with houses of the same 

 general shape, the same height of eaves and the same 

 roof-pitch, the amount of cubic contents is higher in ratio 



