GREENHOUSE 



[passing through such lenses, and it is not difficult to see 

 i that the same effect may be produced by similarly im- 

 f perceptible variations in the thickness of sheet glass. 

 [That this is the case has been conclusively shown by 

 the series of experiments before mentioned. These also 

 (show that burns on plants, caused by defective glass 

 roofs, occur in lines and not in isolated spots, burns 

 |o.f the latter description being usually the result of a 



GREENHOUSE 



691 



FOCAL D'STANCE 10 FT. 

 1EFRACTION *%* -- 



997. Refraction of light rays by an irregular pane of glass. 



weakening or deterioration of tissue, due to carelessness 

 in the matter of ventilation, humidity of the atmos- 

 phere and water, and temperature of Greenhouses, 

 rather than to defects in the glass of roofs. 



If, therefore, it is not possible to obtain glass of uni- 

 form thickness with certainty, it may be found cheaper 

 : and often fully as satisfactory to purchase the lower 

 or common grades of double thick glass, using in the 

 roof only those panes which show, after testing in the 

 sunlight for foci, an entire lack of the prismatic charac- 

 ter which makes them dangerous to plants grown under 

 th em. j. C . BLAIR. 



Greenhouse Heating. In all sections in which the 

 temperature drops below the freezing point, it is neces- 

 sary to provide some artificial means for heating Green- 

 houses. Nearly all modern structures are warmed 

 either by steam or hot water, although hot air flues are 

 occasionally used. While hot water is preferred for 

 small ranges of glass, as it can be depended upon to 



, furnish an even degree of heat when left for a number 

 of hours, steam is more commonly used for extensive 



, plants, as the cost of piping the houses is much less 

 than when hot water is used. Steam boilers require 

 more attention than hot water heaters, but when there 

 is more than 10,000 or 12,000 square feet of glass, it is 

 best to have a night fireman and watchman, and the 

 extra expense can be made up by the saving in the cost 

 of fuel, as it will be possible to use a lower grade of 

 coal. Under these conditions the cost of running a steam 

 plant will be as low as with hot water, but in small 

 houses, where hard coal is used, and the fires receive no 

 attention for six to eight hours during the night, hot 

 water heaters will be cheapest to operate, and will be 

 most satisfactory. See, also, the article Forcing. 



As the various flowers and vegetables grown under 

 glass require different temperatures, the piping of 

 Greenhouses has to be varied accordingly. Thus, al- 



i though it may vary from 3 to 5 for different varieties of 

 the same species, our common plants require the follow- 

 ing night temperature : violets and lettuce, 45 to 50; 

 radishes and carnations, 50 to 55; roses and tomatoes, 



,60; cucumbers and stove plants, 70. 



Boilers. Whether steam or hot water is used for 

 heating, the best boilers for houses with less than 2,000 

 feet of radiation are of cast iron, but for larger houses, 

 especially when steam is used, boilers of a tubular pat- 



. tern are commonly preferred. Although it is not usually 

 practiced, it will be safest and often cheapest in the end 

 if two or more boilers of medium size are used instead 

 of one large boiler of the same capacity as the small 



' ones combined. When only one boiler is used it might 

 result in the loss of all the plants in the house if any 

 accident should happen to it in severe weather, while if 



two or more boilers are used, and are so arranged that 

 any of them can be cut off, the danger from this source 

 will be greatly lessened. The use of two or more small 

 boilers will also be found much more economical than 

 one large one during the fall or spring, when it will be 

 far cheaper to maintain a fire in one of the small boilers 

 than in a large one. 



In selecting a boiler, it is always desirable to have one 

 sufficiently large to afford the necessary heat without 

 forcing the fire, as this will not only give more satisfac- 

 tory results, but will result in the economy of fuel and 

 labor, and will prolong the life of the boiler. Boiler 

 makers generally use some definite ratio between the 

 size of the grate and the amount of fire surface in the 

 boiler, but this varies with the size of the boiler and the 

 efficiency of the fire surface. In small hot water boilers, 

 with very effective fire surface, the ratio between the 

 two is frequently as small as 1 to 15, while in larger 

 boilers it is often as great as 1 to 35, and even more 

 where hard coal is used and the boilers have constant 

 attendance. For small Greenhouses it is desirable to 

 have the grate sufficiently large to permit of leaving the 

 fire without attention for eight to ten hours in the se- 

 verest weather, while for a large range of houses it is 

 customary to employ a night fireman, and a grate much 

 smaller proportionately could be used. In steam boilers 

 the capacity is generally rated at about 100 square feet 

 of radiation for each horse-power; and an average of 

 about 15 square feet of fire surface is considered equiva- 

 lent to a horse-power, it being customary to estimate 

 that 12 square feet in large boilers and 18 feet in very 

 small ones will equal one horse-power. Thus, in boilers 

 of medium size, an area of 10 square feet of grate will 

 answer for 250 square feet of heating or fire surface, 

 and this will be sufficient for nearly 1,700 feet of radi- 

 ating surface, where steam is used; and, as hot water 

 requires about two-thirds more radiation, a boiler of the 

 above size will answer for from 2,800 to 3,000 square 

 feet of hot water radiation. In using the above figures 

 for small boilers that will not have attendance during 

 the night, it is generally advisable to make an allowance 

 for this of about 25 per cent, and, when a boiler is re- 

 quired for 1,000 feet of radiation, select one that would 

 be rated at 1,250 feet. 



For large ranges, tubular steam boilers will generally 

 be more satisfactory. Good results will be secured 

 from those either of fire-tube or of water-tube con- 

 struction, and many prefer them when hot water is 

 used ; but when tubular boilers are used for hot water 

 heating, although good results may be secured when 

 a regular steam boiler is employed, it is advisable to 

 have them made without a steam dome, and to have the 

 entire shell filled with tubes (Fig. 998). As a rule, these 

 boilers will be less expensive than cast-iron boilers, and 

 if properly cared for, will be nearly as durable. 



During the past few years a large number of coil 

 boilers have been constructed for hot water heating. 

 These boilers are generally from 4 to 6 feet long, and are 



998. Horizontal tubular boiler for hot water. 



made from wrought-iron pipe, varying in size from 1 to 

 2 inches in diameter, but when constructed from 1-inch 

 pipe they are not very durable, as the pipe itself is 

 comparatively thin, and wherever the threads are ex- 

 posed it is quickly eaten through. There is also more 

 trouble from the boiling over of the water than when 

 larger pipes are used, and when boilers are constructed 

 of 1-inch pipe it is necessary to have either an elevated 

 expansion tank or to run it as a closed system. In 

 making the boiler the pipes are cut of the desired 

 length, usually of 5 or 6 feet, and the ends are con- 

 nected either by return bends or by manifolds, so as to 



