332 VENTILATION WITH FANS. 



eflTect of the liighly-heated air in the upper regions must be 

 obviou:>. W'^ have measured the temperature of a house 45 

 feet in heighi.and have found the temperature at the floor of the 

 house to be 3S°, while the temperature of the upper stratum was 

 103°, showing a difference of 65°. In many other cases, we 

 have found the temperature of the upper stratum of air in a 

 house, above 120°, while the water cistern, at the floor of the 

 house was covered with ice. The application of a fan may be 

 beneficial in reducing this temperature, and expelling the foul 

 air collected in the upper portions, at apertures lower down the 

 house. 



Various other mechanical contrivances, besides the fan, have 

 been used for producing motion in the atmosphere of houses. 

 Among these may be mentioned common windmills, of which 

 we have already spoken. The windmill ventilator is a very 

 adaptable machine, and may be constructed very simply, in con- 

 nection with a hot-house, and applied in moving the atmosphere 

 of the house, or in propelling the warmed air through the con- 

 duction tubes with greater velocity than it would acquire by its 

 own specific gravity. The windmill, of course, is turned by 

 the force of the wind outside the house, and is entirely depend- 

 ent upon the motion of the external air, for the power it exer- 

 cises over the internal atmosphere. In hot-houses, with dome- 

 shaped roofs, it is well adapted for drawing ofl" the highly- 

 heated air at the top of the house, and may be made something 

 like the screw propeller of the steamboats, and situated directly 

 in the apex of tho roof. 



Pumps have also been used for drawing off the foul air from 

 buildings, although we are not aware that they have ever been 

 employed for ventilating hot-houses, for which they are not at 

 all adapted. 



Chimney shafts are well adapted for producing motion in the 

 air, by the draughts. None of these methods, however, are so 

 useful as the fan, when mechanical means are to be applied ; 

 though, for the practical purposes of ventilation, in horticultural 

 structures, the common process of spontaneous ventilation must, 

 in general cases, suffice ; — and, therefore, the question is, as to 

 the means of admitting the air, and the temperature at which it 



