Retrospective Criticism. 5 I 7 



" 243. To measure the quantity of elastic vapour in the atmosphere, Hy- 

 grometers have been invented, and the degree of moisture is indicated in 

 these instruments by what is called the dew-point. The best hygrometer is 

 that of Daniell ; but, as some nicety is required in its use, a substitute has 

 been found in two common thermometers," &c. 



A thin tumbler, as described in Gard. Mag. for July, 1842, p. 367., is the 

 hygrometer most easily managed and understood. 



" 268. A sensible effect on the human feelings produced by the atmosphere 

 of hothouses heated according to Mr. Penn's principle is, that a high tem- 

 perature, say of 80° or 90°, can be breathed in as agreeably, and for as long a 

 period, as one of 60° or 70° not in motion. This result is partly attributed to 

 the motion given to the air ; since, in the hottest days of summer, the heat, 

 which would be oppressive in still air, is rendered not only bearable but 

 even agreeable, if the air is put in motion by a breeze. In like manner the 

 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." 



The motion of air or wind is caused by colder air replacing warmer ; this 

 may cause the cooling effect of breezes in summer. Why the effects of still 

 cold air are not so great as those of air in motion is, because, when in motion, the 

 cold air is constantly replacing that partially heated by the human body. 

 Why motion of heated air should, when uniformly heated, give relief is not 

 so plain. Why moisture gives relief is connected with electricity. In dry air 

 the electricity of the body accumulates, because dry air is a bad conductor. 

 Moist air, being a good conductor, draws off the excess of electricity, which, 

 when present, was causing a pricking uneasy sensation ; and, when removed, 

 the body gets more elastic and exhilarated. Motion is undoubtedly of benefit 

 to leaves and stems of plants. 



"281. When light falls on a transparent medium, a portion of the rays is 

 transmitted through it, and a portion is reflected from its surface. The latter 

 portion follows the same laws as the light which is reflected from opaque 

 surfaces ; and the portion which passes through it is refracted, that is, it 

 leaves the transparent medium at a different angle from that at which it fell 

 upon it ; and by this change the light is also weakened, so as at a very short 

 distance from the surface of the transmitting medium, as of glass, for example, 

 to be dispersed and transfused in the atmosphere, in which state in hothouses 

 it has no longer the same power on the vital energies of plants. We are not 

 aware that the cause of the inefficiency of light, after it has passed through 

 glass and reached a certain distance, has been fully explained ; but the fact is 

 well known to gardeners, who, in hothouses, invariably place the plants they 

 wish to thrive best at the shortest distance from the glass. As the quantity 

 of light which passes through glass at the roof of hothouses is, all other 

 circumstances being the same, greatest when the plane of the roof is at right 

 angles to the plane of the sun's rays ; hence, the slope of the roof is, or ought 



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