78 



GARDEN WALLS. 



be, all other circumstances being equal. 

 A dark-coloured wall absorbs more, and 

 reflects less, than a light-coloured one; 

 but in the case of either dark or light 

 coloured, the amount of heating rays im- 

 pinging on them is the same. Reflected 

 solar rays, being of less intensity than 

 those that are direct, do not scorch the 

 bark of trees ; and with still less reason 

 can such an effect be attributed to the 

 gradual radiation of heat from your 

 blackened wall. Direct solar rays will 

 raise a thermometer, placed against a south 

 wall, as much as 100° Fahr. above the 

 freezing-point, and the exposed parts 

 only of naked stems are frequently 

 injured. The parts of such stems as are 

 next the wall, and consequently most 

 exposed to its radiation, have not been 

 noticed to be injured, so far, at least, as 

 our observations have extended : nor is 

 it likely ; for the heat from this source 

 will rarely, if ever, be equal to 50° above 

 the freezing-point, or half as much as 

 that occasioned by direct solar rays." It 

 clearly appears that the colour of walls 

 is practically of no importance what- 

 ever, although in theory it may seem to 

 be so. The best colour, therefore, is that 

 of well burned bricks for new walls, and 

 that of Mulgrave cement for such old 

 ones as have been repaired. 



"The radiation during night and in 

 cloudy weather," says Loudon, "is in 

 proportion to the absorption during sun- 

 shine ; the one operation neutralises the 

 other. If, indeed, we could insure a 

 powerful absorption from a bright sun 

 during the day, and retain the radiation 

 by a canvass or other screen during the 

 night, a considerable increase of tempera- 

 ture might probably be the result ; but 

 the number of cloudy days in our cli- 

 mate, 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 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." If there be any 



colour worse than another, it is certainly 

 that of cherry red bricks, possessing in 

 themselves no extra property of absorb- 

 ing heat from the sun, and of all colours 

 harmonising least with the surrounding 

 scenery. The nearer, therefore, bricks 

 are brought to a subdued stone colour — 

 and this can be readily done in the making 

 — so much the better. It was long sup- 

 posed that colour had great influence on 

 radiation and absorption. By exposing 

 variously coloured surfaces to the heat of 

 the sun, their absorbing power was found 

 to be in the following order — black, blue, 

 green, red, yellow, and white. Hence it 

 would naturally be expected, that the 

 radiating powers of differently coloured 

 bodies would be in this order, and that 

 by painting a body of a dark colour we 

 should increase its radiating power. 

 Such, however, is not the case — for the 

 absorption and radiation of simple heat, or 

 heat without light, depends on the nature 

 of the surface rather than on colour. 

 Heat of low temperature, or that which 

 proceeds from bodies at a low tempera- 

 ture, becomes less connected with colour 

 the lower the temperature. Hence sun 

 heat, which is at a low temperature in 

 northern latitudes, can have little greater 

 effect on a dark-coloured wall than on 

 walls of other colours. 



§ 9. — CONSTRUCTION OF GARDEN 

 WALLS. 



Garden walls are variously constructed 

 with a view to economy, utility, and 

 effect — as the following examples will 

 sufficiently show. 



The solid brick wall. — This is the most 

 common as well as the simplest of all 

 walls ; and where the height does not 

 exceed 6 or 7 feet, 9 inches — that is, the 

 length of a common brick — will suffice 

 for the thickness, this being found suffi- 

 ciently strong without the aid of piers. 

 When from 7 to 12 feet in height, 14 

 inches— that is, the length of one brick 

 and breadth of another — will be required ; 

 walls of greater height ought to be 18 

 inches thick, or the length of two bricks. 

 Such walls are found to be sufficiently 

 strong even when hothouses are built 

 against them, as they have a resisting 

 support from the necessary sheds and 



