PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. 313 



ing to the wind, or other circumstances likely to affect the inter- 

 nal atmosphere. Then small apertures may be left open in 

 different parts of the house, during the night, whereby an inter- 

 change of the atmospheric volume would take place, without 

 exposing the plants to immediate contact with the cold air. By 

 this plan, we conceive that direct benefit would accrue to the 

 plants, because the air between the covering and the glass, 

 although not cold, would nevertheless be of greater density than 

 that of the house, and would consequently find its way into the 

 interior, by the ventilators left open for that purpose. This 

 would also enable us to maintain a much lower night tempera- 

 ture than could possibly be otherwise done, with regard to the 

 safety of the plants, which the fear of sudden changes during the 

 night, and consequent injury from frost, prevent from being 

 realized in this changeable climate. 



It is truly remarkable how very slight a covering is required 

 to exclude a pretty severe frost. " I have often," observes Dr. 

 Wells, " in the pride of half-knowledge, smiled at the means fre- 

 quently employed by gardeners to protect tender plants from 

 cold, as it appeared to me impossible that a thin mat, or any 

 such thin substance, could prevent them from attaining the tem- 

 perature of the surrounding atmosphere, by which alone, I thought 

 them liable to be injured. But when I had learned that bodies 

 on the surface of the earth, become, during a still and serene 

 night, colder than the atmosphere, by radiating their heat to the 

 heavens, I perceived immediately a just reason for the practice 

 which I had before deemed useless. Being desirous, however, 

 of acquiring some precise information on this subject, I fixed 

 perpendicularly in the earth of a grass plot four small sticks, and 

 over their upper extremities, which were six inches above the 

 grass, and formed the corners of a square, the sides of which 

 were two feet long, fixed a thin cambric handkerchief, so as 

 to cover the included space. In this disposition of things, there- 

 fore, nothing existed to prevent the free passage of air from the 

 surrounding grass to that which was sheltered under the hand- 

 kerchief, except the four small upright sticks supporting it, and 

 there was no substance to radiate heat downwards to the grass 

 beneath but the cambric handkerchief. The temperature of the 



