Retrospective Criticism. 519 



and healthier, and the extremes between heat and cold not so great. In 

 Scotland, woollen nets are most used; from the coldness of the climate, 

 they are most beneficial ; and those who keep them constantly standing find 

 they do harm ; the foliage is not so healthy, and insects collect. There 

 is seldom so much heat there as to require shading for the blossom. Dry 

 cold east winds do most harm. 



" 474. Colouring the surface of walls black, with a view to the absorption 

 of heat, has been tried by a number of persons, and by some it has been con- 

 sidered beneficial ; but, as the radiation during night and in cloudy weather 

 is necessarily in- proportion to the absorption during sunshine, the one 

 operation neutralises the other. If, indeed, we could insure a powerful ab- 

 sorption from a bright sun during the day, and retain the radiation by a 

 canvass or other screen during the night, a considerable increase of tem- 

 perature might probably be the result ; but the number of cloudy days in our 

 climate, in proportion to those of bright sunshine, is not favourable to such an 

 experiment." 



White walls will heat the air around the leaves most through the day from 

 reflection, as these are seldom close to the wall ; and the extreme of cold will 

 not be so great at night, which is most dangerous. Black-coloured walls, 

 though they absorb heat during the day, will not retain it to give off at 

 night, as it will be conducted through the wall, in great part, during the day, 

 and any little retained be speedily radiated off in the early part of the night. 



" 500. Whatever mode of heating or kind of pipes may be adopted, the 

 pipes should always have a gradual ascent from the place where they enter 

 the house, or are intended first to give out heat, towards the farther ex- 

 tremity ; otherwise, the circulation will be less rapid, and consequently the 

 heat less equally distributed." 



Practically this is not found to be the case. The reason is, that the force of 

 the gravity of the cold water in the returning pipes is increased by the height 

 gained, the escape of air at the highest point, &c. (See Hood's Treatise, p. 18.) 



"501. To explain the manner in which the motion of heated air in hot- 

 houses produces a sensation of coolness, without being altered in its tem- 

 perature, we make the following quotation from Lardner's Cyclopedia : 

 ' The air which surrounds us is generally at a lower temperature than that of 

 the body. If the air be calm and still, the particles which are in immediate 

 contact with the skin acquire the temperature of the skin itself, and, having 

 a sort of molecular attraction, they adhere to the skin in the same manner as 

 particles of air are found to adhere to the surface of glass in philosophical 

 experiments. Thus sticking to the skin, they form a sort of warm covering 

 for it, and speedily acquire its temperature.' Agitation of the air, however, 

 ' continually expels the particles thus in contact with the skin, and brings new 

 particles into that situation. Each particle of air, as it strikes the skin, takes 

 heat from it by contact, and, being driven off, carries that heat with it, thus 

 producing a constant sensation of refreshing coolness.' " 



The temperature of the blood is 94° to 98°, and the heated air is not likely 

 to be much below the temperature of the skin ; to that extent, however, it will 

 undoubtedly increase the effect ; and, in motion, will give motion to the leaves 

 and stems of plants, and will not stagnate and corrupt. 



" 504. Rogers's conical boiler and hot-water apparatus? Why should a 

 2-inch pipe ascend, and 4-inch pipe descend ? the friction will be in much 

 greater proportion in the 2-inch pipe, and the molecular ascent from heat have 

 more to contend with. 



" 564. Various experiments have been made to accelerate germination, with 

 different degrees of success. These all proceed on the principle that ger- 

 mination cannot take place until the carbon of the seed is changed into 

 carbonic acid ; and this can only be done by extraordinary supplies of 

 oxygen," &c. 



Substances yielding oxygen should be of most use in germination to oily 

 seeds, which have a deficiency of oxygen in themselves. 



LL 4 



