290 MANAGEMENT OF THE ATMOSPHERE. 



shown by the arrows in the foregoing ficrure. By this method, 

 air may be introduced into a house at any period of the day, or 

 even at night; and while every advantage arising from the 

 admission of external air is gained, the disadvantages are done 

 away with, save and except by the crevices in the structure. 

 In winter, if cold air imist be introduced to regulate the internal 

 temperature, some such method as that given above should be 

 adopted; but at a more advanced season of the year, when a 

 larcrer supply of air is necessary, provision nmst be made at the 

 sides for that purpose. As to opening the top sashes first, and 

 keeping them open till the last, it is a practice for which we are 

 unable to obtain any satisfactory reason, and which we think 

 will not bear a strict investigation. But, it may be asked, how 

 is the temperature to bs reduced, where, at an advanced period 

 of spring, the sun shines more powerfully, and when the tem- 

 perature of a hot-house will suddenly rise ten or fifteen degrees 

 above the maximum point? To answer this question, it is 

 necessary to consider whether there be any other method of 

 reducing the temperature than by expelling the heated air, by 

 the opening of the top sashes. From what has already been 

 said on this point, we think we are fully justified in disposing 

 of this question in the affirmative. Of course, we do not allude 

 to the ventilation of houses in summer, but in the months of 

 autumn, winter, and spring. By introducing the external air in 

 the manner described in the last figure, the atmosphere of a hot- 

 house will be reduced to any given point as effectually, though 

 not so rapidly, as if the heated air was expelled through the 

 sashes at the top of the house. This is accounted for by the 

 circumstance already explained, viz., that when two columns of 

 air of unequal temperatures are mixed together, the tempera- 

 ture of the whole is reduced, while its density is increased ; and- 

 hence, so long as the atmosphere continues to be heated by 

 reflection or radiation, this cold air will continue to cool it down, 

 so that nothing is lost, while all the essentials of vegetation 

 contained in the atmosphere are retained. 



5. The materials of which the internal part of the house is 

 composed have also a powerful influence on the ventilation of a 



