690 



GREENHOUSE 



GREENHOUSE 



desirable to open all the ventilators in a long house with 

 one set of apparatus, for frequently one end will not 

 need as much ventilation as the other end, or may be 

 affected by the wind forming a current lengthwise of 

 the house. To avoid this a Greenhouse 200 feet long 

 should have 3 or 4 sets of apparatus, which can be ope- 

 rated separately. In all Greenhouses of considerable 

 width, it is desirable that ventilation should be provided 

 on both sides of the ridge, so that the ventilation can be 

 given on the "leeward" side, which will prevent the 

 wind from blowing directly into the house. 



Heating. The success of the florist, gardener or 

 amateur in the management of a Greenhouse depends 

 largely on the satisfactory working of the heating appa- 

 ratus. There are two systems of Greenhouse heating 

 which, when the apparatus is properly installed, are 

 economical and satisfactory; viz., hot water and steam. 

 The open-tank hot water heating has more advantage 

 in its adaptation to general use than any other, and is 

 so simple that its management is readily understood by 

 any one. It is practically automatic, and is capable of 

 maintaining an even temperature for ten hours without 

 attention. Low pressure steam heating is well adapted 

 to large commercial ranges, and to large conservatories 

 in parks and private places, where a night attendant can 

 be kept in charge of the fires to turn on and shut off 

 steam from the radiating pipes as the changing outside 

 temperature may require. The heating of Greenhouses 

 to the best advantage under the varying .conditions of 

 climate and interior requirements, demands, like the 

 designing of Greenhouses, the services of an experi- 

 enced specialist in horticultural work. 



LORD & BURNHAM Co. 



Greenhouse Glass. The selection of glass for Green- 

 houses, and the nature of the imperfections which ren- 

 der it undesirable for such use, are questions which 

 have received much attention from horticultural writers, 

 and which have brought forth a variety of answers. 

 Three qualities are essential in all glass to be used in 

 Greenhouse construction: first, minimum of obstruction 

 to solar rays ; second, strength sufficient to withstand 

 the strain of winds and storms, especially hail ; and 

 third, freedom from defects rendering it liable to burn 

 plants "grown under it. 



It is an established fact that plants thrive best under 

 a clear and transparent glass, which lets through the 

 greatest possible percentage of the sun's rays. This 

 includes all the solar rays, calorific or heat rays and 

 actinic or chemical rays, as well as the colorific or light 

 rays. Clear white glass of the grade known as "single 

 thick" (12 panes to the inch) lets through from 60 to 70 



995. Details of iron-frame benches. 



per cent of the sun's rays, common green glass of the 

 same thickness, 52 to 56 per cent, and "double thick" 

 (8 panes to the inch) common green glass from 50 to 52 

 per cent. This percentage is reduced by other colors, 

 dark blue glass letting through but 18 per cent. In con- 

 nection with the matter of tint, it should be noted that 

 some glass, especially clear white glass purified with 

 arsenic acid, or that in which a large amount of potash 

 is used in proportion to the amount of lime used in 

 manufacture, becomes dull after long exposure to the 

 weather, the dullness being occasioned by the efflo- 

 rescence of salts contained in the glass. Before this dis- 

 integration has proceeded too far, the crust or efflo- 

 rescence may be removed with muriatic acid. 



The strength of glass depends upon its thickness and 



996. Burned areas on a 

 Begonia leaf. 



the thoroughness of the annealing. Glass is annealed 

 by passing through a series of ovens, where it is raised 

 to a high heat and then gradually cooled, whatever 

 toughness and elasticity the finished product may con- 

 tain being due to this pro- 

 cess. The thickness of glass 

 varies, not only with grades 

 (single and double thick), 

 but also more or less within 

 the grades, and even with 

 different parts of the same 

 pane. Single thick glass is 

 too thin for use in Green- 

 houses, and in selecting any 

 glass for such a purpose it 

 should be examined pane by 

 pane, and all showing 

 marked variation in thick- 

 ness, either between panes 

 or in different parts of the 

 pane, rejected. A pane of 

 varying thickness is much 

 more liable to breakage from 

 climatic changes or sudden 

 shocks than one which is 

 uniform in this regard. 

 Prom the foregoing state- 

 ments it will be seen that, in 

 general, the ordinary dou- 

 ble thick green glass is best, 

 as regards both tint and 

 strength, green glass being 

 less liable to change in tint than white, and the double 

 thick being the stronger grade. By green glass is meant 

 simply the ordinary sheet glass, the green color of 

 which is only noticeable when looking at a cut edge. 



The idea has long been more or less prevalent that 

 such visible defects in sheet glass as the so-called 

 "bubbles," "blisters" and "stones," produce a focusing 

 of the solar rays passing through them, thus burning the 

 foliage of plants grown under glass containing these 

 defects (Fig. 996). This view has been held by glass 

 manufacturers and horticulturists alike, and seems not to 

 have been publicly contradicted until 1895 (Bull. 95, Cor- 

 nell University Agric. Exp. Sta., p. 278). In view of the 

 erroneousness of this theory, it is rather remarkabh 

 that it should have gained such prevalence. Nearly all 

 bubbles and blisters are thinner in the middle than a' 

 the periphery, being thus concave rather than conve> 

 lenses, and actually diffusing the rays of light passing 

 through them rather than producing destructive foci 

 While it is true that sand stones or knots in glass ma} 

 produce foci, these points of focus scarcely ever exis 

 more than a few inches from the surface of the glass 

 and consequently these defects can do no damage whei 

 occurring in roofs several feet distant from the grow 

 ing plants below. 



The only full and complete series of experiments 01 

 this subject in this country (conducted at the Cornel 

 University Agricultural Experiment Station, the Physi 

 cal Laboratory of Cornell University, and a glass fae 

 tory in Ithaca, New York, but yet unpublished) show 

 the true cause of the burning by glass to be th 

 variation in thickness of the entire pane, or poi 

 tion of same, thus causing a prismatic or lens-like el 

 feet (Fig. 997), which causes a more or less distinc 

 focussing of the sun's rays at distances varying fror 

 5 or 6 feet to 30 feet, or even more, from the glass. 



This defect usually occurs along the side or end < 

 the pane, and is not visible to the eye, but may '> 

 easily detected by the use of the micrometer caliper 

 by testing in the sunlight. It may be found in all kii 

 of glass, and is caused by a reduction of the upper 

 pipe end of the cylinder from which sheet glass is ms 

 by the glass blower, to facilitate the removal of 

 "cap "or neck end of the cylinder, by which it is 

 tached to the pipe while being blown. The defect, 

 before stated, is one which may be found in all gr 

 and qualities of sheet glass, of both foreign and don 

 tic manufacture. The fact is well known that dil 

 ences in the thickness of spectacle lenses, which 

 imperceptible to the eye, may produce sufficient ret 

 tion to materially vary the direction of rays of lij 



