86 



AGITATION OP THE ATMOSPHERE, 



absence of heat is much more severely felt when the air is in motion, than 

 when it is at rest. Captain Parry and his companions, when in the Polar 

 regions, could endure a degree of cold when the air was still, that, when it 

 was put into motion, they found to be quite intolerable. It is certain, how- 

 ever, that a part of the agreeable effect produced by the motion of the air in 

 Mr. Penn's hothouses is owing to the moisture which it contains ; for the 

 human feelings in a hothouse heated to 80, in which no attempt has been 

 made to saturate the air with moisture, are much less agreeable than in one 

 at the same temperature in which the paths are kept moist with water. 

 Every one must be aware of this who has felt the heat of a stove heated by 

 brick flues, as compared with one heated by hot water ; for though no water 

 may escape from the pipes to moisten the air, yet no moisture is absorbed by 

 them from the air of the house. In a house heated by flues, on the contrary, 

 the clay of the bricks in the flue covers, and the lime by which the sides of 

 the flues are plastered, having, as we have seen (155 and 156), a great che- 

 mical attraction for water, abstract it from the air of the house, and give it 

 that peculiar dryness which is so unpleasant to the skin, and so oppressive 

 to the lungs. Alluding to this dry heat, Mr. Daniell says : 



269. " To the human feelings the impression of an atmosphere saturated 

 with moisture is very different from one heated to the same degree without 

 this precaution ; and any one coming out of a house heated in the common 

 way, into one well charged with vapour, cannot fail to be struck with the 

 difference. Those who are used to hot climates have declared that the feel 

 and smell of the latter exactly assimilate to those of the tropical regions." 



270. The excellence, Mr. Rogers observes, " of Mr. Penn's method of 

 warming and ventilating buildings appears to consist in the very uniform 

 degree of moisture which it produces in the atmosphere. The heated air 

 which enters the hothouse has already received a dose of moisture nearly 

 sufficient to saturate it, and has not to seek its moisture among the plants, as 

 is generally the case. In most plant houses the pipes are placed under the 

 front shelves, at a considerable distance from the floor, and the atmosphere 

 is moistened by syringing the plants, or throwing water on the floor and 

 shelves. How greatly the state of an atmosphere so produced differs from 

 that of Mr. Penn's houses, a little consideration of the annexed sketch will 

 show. It is the section of a house heated by pipes under the front shelves ; 

 and it must be borne in mind that the capacity of air for moisture varies 

 with its temperature, so that air which was saturated at 56, becomes very 

 dry when heated to 70. 



271. The sketch fig. 3 is the 

 section of a house heated by pipes in 

 the ordinary manner, under the front 

 shelves. The arrows (numbered) 

 indicate the course of the current. 

 of air. At No. 1 the air comes 

 heated from the pipes p, and ex- 

 tremely thirsty; at No. 2 it find;* 

 moisture among the plants, and. 

 rising from the damp and warm 

 shelf (slate, of course) ; at No. 3 it 



has parted with some of its heat ; it Fig. 3. Section of a Hothouse heated by hot u-ater 



is now supersaturated, and is parting in the ordinary manner. 



