690 



GREENHOUSE 



desirable to open all the ventilators in a long house with 

 one set of apparatus, for freciuently 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 i)revt;nt the 

 wind from blowing directly into the hnu-,.-. 



Meating.-The success of the floii-i. L'i:<!it.. i .r 

 amateur in the management of a Gn-fiiliiu .■ d, |m nl, 

 largely on the s;ttisf;u-turv wnrliiiii^ of i li-' Im' i : ;ii _■ ,111:1- 

 ratus. Thei-f in-- f-v.i --.--t.-i,,. .,f i ;;■.. ■!,),.. , . 1 . i,';, 

 which, when 1 ' ■ i !•■■■■■• ■ ' ':■"'■!. 



economical am! 



The 



open 



in its adapi;ih-ii tu ^i uliuI 

 so simple that its m:iu;igemL-i 

 any one. It is practically au 



attention. Low pressure ste: 

 to large < 



turn on and shut off 



Loud & Bukxh.vm Cu. 

 Greenhouse Glass. -The selection of glass for Green- 

 of the imperfectio! 



fhich 1 



houses, and the 



der it undesirable for such n: 



have received much attention fniiii li.uti.ultui;.! urlt.rs. 



and which have brought fortli ;i v.n-fty ni :ni-.M r~. 



Three qualities are essential in all ^lass t,. I,, n-.-.i nj 



Greenhouse construction: first, ininiinuni cil' (.l.striictinn 



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 

 a ^tinic 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 



frame benches. 



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

 same thickness, 52 to 50 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 



GREENHOUSE 



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 - r-,' , , 

 (single and double thick 1. L^ 

 but also more or less within ^Si 

 the grades, and even with ^ 

 different parts of the same '^" 

 pane. Single thick glass is 

 too thin for use in Green- 

 hiMises, and in selecting any 

 L'li 5 for such a purpose it 

 t ';li lie examined pane by 

 , and all showing 

 In I! I.. (I variation in thick- 

 li' . either between panes 

 iir 111 different parts of the 

 p.uir, rejected. A pane of 

 varying thickness is much 

 iiiiire liable to breakage from 

 climatic changes or sudden 

 shucks than one which is 

 uniform in this regard. 

 Fnini the foregoing state- 

 iri' iits it will be seen that, in 

 LI ii.ral, the ordinary dou- 

 li!- Ill irk green glass is best, 

 a, ii-ards both tint and 

 .^ll■^;u^'th, green <rla'^< K.-iiii,' 



less liable to cliaiiL'-' in Imt lliali wIiIIn. an.l ilir .Inuble 

 thick being thi- -I I . 1 1. , nuant 



simply the onlin.i ii. • 1 li ■.> i -I.ir of 



which is only ui.ti.a .iMi ^ h. n |....|,ii.l ,,1 ;, , m , ,i_r,.. 



The idea has Ion;,' ln-i-n more or li-ss pn-vak-nt 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 (Pig. 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 remarkable 

 that it should have gained such prevalence. Nearly all 

 bubbles anil blisters are thinner in the middle than at 

 the periphery, being thus concave rather than convex 

 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 may 

 produce foci, these points of focus scarcely ever exist 

 more than a few inches from the surface of the glass, 

 and consequently these defects can do no damage when 

 occurring in roofs several feet distant from the grow- 

 ing plants below. 



The only full and complete series of experiments on 

 this subject in this country (conducted at the Cornell 

 University Agricultural Experiment Station, the Physi- 

 cal Laboratory of Cornell University, and a glass fac- 

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

 the true cause of the burning by glass to be the 

 variation in thickness of the entire pane, or por- 

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

 fect (Fig. 997), which causes a more or less distinct 

 focussing of the sun's rays at distances varying from 

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



This defect usually occurs along the side or end of 



the pane, and 

 easily detected by tin- i 

 by testing in tin- sunli- 

 of glass, and is laii^. ■! 

 pipe end of the i-\ lln.l.i 

 by the glass bhiuxr, t 

 "cap" or neck end of 



■isil 



eye. but may he 

 f tin- micrometer caliper or 

 It may be found in all kinds 

 I n iliii-tion of the upper or 

 I wliirh sheet glass is made, 

 rilitate the removal of the 

 der, by which 



tached to the pipe while being blown. The defect, as 

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

 and qualities of sheet glass, of both foreign and domes- 

 tic manufacture. The fact is well known that differ- 

 ences in the thickness of spectacle lenses, which are 

 imperceptible to the eye, may produce sufficient refrac- 

 tion to materially vary the direction of rays of light 



