THE ANGLE OF ELEVATION. 



139 



" Light, to a certain extent, follows the 

 same laws as heat. It is received by 

 radiation from the sun, reflected by 

 smooth surfaces, transmitted and refracted 

 by transparent substances, such as water 

 and glass ; concentrated by reflection from 

 concave surfaces, and dispersed by re- 

 flection from surfaces that are convex. 

 Light, however, differs from heat in the 

 impossibility of retaining it after the 

 absence of the sun ; whereas heat can be 

 retained by enclosing heated bodies in 

 non-conducting mediums, and by reflect- 

 ing it back to the surfaces from which it 

 is radiated. The radiation of light is 

 greatest when the radiating rays strike 

 the surface at a right angle, and the least 

 when the angle is most oblique ; because, 

 in the former case, the rays are reflected 

 on every side, and consequently the sur- 

 rounding objects are illuminated propor- 

 tionably ; and in the latter case the 

 greater number of rays pass off at one 

 side, and illuminate less effectively the 

 surrounding medium. The reflected rays 

 are always returned from the surface on 

 which they radiate, at an angle equal to 

 the angle of incidence : if the reflecting 

 surface be a plane, the reflected rays will 

 be parallel to each other ; if the surface 

 be convex, they will be divergent, and 

 consequently dispersed ; and if it is con- 

 cave, they will be convergent, and hence 

 concentrated. Smooth and shining sur- 

 faces reflect most light, and rough and 

 dark surfaces least ; and with respect to 

 colour, white reflects almost all the rays 

 of light which fall upon it, and black 

 absorbs them all. 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 trans- 

 parent medium at a different angle from 

 that in 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 trans- 

 fused 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 dis- 

 tance, 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, so the slope of the roof 

 is, or ought to be, adjusted to the direc- 

 tion of the sun's rays at that season of 

 the year when its light is most wanted. 

 As in houses for early forcing, the greatest 

 deficiency of solar light is in the winter 

 season, when the sun is low, so the roofs 

 of such houses are made steep, in order 

 that the sun's rays may be received at a 

 larger angle. Summer forcing-houses, on 

 the other hand, have less steep roofs, so as 

 to receive most benefit from the sun in 

 April, May, and June, when forced fruits 

 are ripening. A greenhouse in which no 

 fruit is ripened, but in which abundance of 

 light is required all the year, has commonly 

 perpendicular glass to receive a maximum 

 of light during winter ; and a sloping roof 

 of glass at an angle of 45°, which is found 

 favourable for the admission of light at 

 every season, as well as for throwing off 

 rain. " — /Suburban Horticulturist. 



After all that has been said on the angle 

 of elevation, it is not at all strange that no 

 particular angle should be fixed upon as 

 the true one ; because that which would be 

 a very proper angle for a peach-house, for 

 instance, at Torquay, would be a bad one at 

 Thurso, the difference of latitude beingcon- 

 siderable. That for early forcing, anywhere, 

 would be improper for a late forcing-house 

 in the same latitude. It therefore follows 

 that the angle of elevation, if of the im- 

 portance some attach to it, and which to a 

 certain extent we do not deny, must be 

 made to correspond with the latitude the 

 house is placed in, and also to suit the 

 purpose for which that house is intended. 



The following may be given as an ex- 

 ample in addition to what has been said 

 above. To obtain the perpendicular rays 

 of the sun in December, it would be neces- 

 sary, in latitude 53°, to place the glass at 

 an angle of 75° 28' ; in January, 71° 52' ; 

 in February, 62° 29' ; and in March, 51° 

 41' ; and so on, if the object is to secure 

 the greatest amount of solar influence. 



