711 



HOTHOUSE. 



HOTHOUSE. 



742 



of a house fronting due south, the rays of the sun at 6 A.M. will run 

 parallel with the roof ; at 7 A.M. 30 per cent, of the rays will be re- 

 flected from the glass alone, independent of the quantity from other 

 causes. At 8 A.M. 10, and at 9 A.M. about 1 per cent, will be reflected. 

 From this time till 3 P.M. the proportion reflected will be only from 

 2 to 1 per cent. If we therefore take from 6 A.M. to 6 P.M., a length 

 of time from which the tropical days never differ much, we may cal- 

 culate the loss of rays to be from 



Reflection of glass, say on an average 

 Obstruction from rafters (iron) . 



Ditto from glazing laps 



10 per cent. 



8 

 6 



23 per cent. 



But if wooden rafters are employed, 15 per cent, additional must be 

 added, making in this case a loss of 38 per cent. In old and heavily 

 constructed houses, it certainly would not be too much to state the 

 loss of rays at 50 per cent. ; and under this privation of light it is not 

 surprising if the plants are found to exhibit a yellow sickly foliage. 



The above calculation of the reflection of rays is made on the suppo- 

 sition that the pitch, or elevation of the roof forms an angle with the 

 horizon corresponding with the latitude of the place, say for London 

 51 J, or forming an angle with the back wall equal to the complement 

 of the latitude, or 384. The period of the season is that of the spring 

 or autumn equinox. A plane elevated to the above angle would have 

 the sun's meridian rays more nearly perpendicular throughout the 

 year than could be the case with any other elevation. This however 

 is not the most eligible angle of elevation, except perhaps in the case 

 of a very early forcing vinery or peach-house, where the direct rays of 

 the sun are more especially required at an early period of the season. 

 The principal objection to this elevation is, that it occasions the house 

 to be built too high in proportion to its width, and the heat accumu- 

 lates in the upper angle where it is least wanted ; in short, the higher 

 the back wall, the colder is the air of the house at its base, and the 

 warmer at the top, compared with the mean temperature throughout 

 the house. It will therefore be proper to inquire how far the above 

 elevation may be deviated from without greatly affecting the trans- 

 mission of light. If the slope of the roof were lowered so as to form 

 an angle of 20 with the horizon, the loss from reflected rays at noon 

 would average about 3 per cent. ; but in the morning and afternoon 

 the loss would be considerably greater, more especially in the winter 

 season ; and it is of course proportionally lees in summer. So far 

 therefore as light is concerned, any angle may be chosen that ig found 

 in other respects the most convenient between 20 and 50. Pine pits 

 and frames are even lower than 20 ; but with reference to the supply 

 of light, as well as the descent of the rain, it is not advisable to have 

 the roof flatter than 20. 



Although the necessity of admitting as much light as possible is now 

 generally acknowledged, and although in cloudy weather great advan- 

 tage will be derived from a roof rendered as transparent as possible, 

 yet the generality of tropical plants do not thrive in bright sunny 

 weather when placed near the glass of a hothouse in this climate ; and 

 the necessity of a screen of netting will appear obvious from the effecte 

 of solar radiation observed in the tropics as compared with, what is 

 rather surprising, the still greater energy of the same in this country. 

 In the tropics, a thermometer covered with black wool, and conse- 

 quently prepared to receive the full effects of the sun's rays, does not 

 rise higher than one so exposed in this climate frequently does, not- 

 withstanding the greater general coldness of the air. It is not uncom- 

 mon in the neighbourhood of London for a thermometer placed in the 

 sun to rise 50 Fahr. above one in the shade. At Cumana, Humboldt 

 never found the sun's rays to have the effect of raising the thermometer 

 more than 6 or f". Nearer the pole, the energy of the solar rays 

 appears to be still greater than in this climate. Between lat. 80 and 

 81, Captain Scoresby states that the thermometer was 18 below 

 freezing on one side of the ship, whilst on the other the pitch was 

 heated to a temperature of 90 or 100. This is a greater radiating 

 effect than has perhaps ever been observed in this climate, and certainly 

 unequalled in the tropics. The body of the atmosphere surrounding 

 the earth is supposed to have the form of an oblate spheroid, flattened 

 at the poles and elevated at the equator. Indeed if this were not the 

 case, as it is found to be considerably denser at the poles than at the 

 equator, a different barometrical indication would be the consequence ; 

 but at the level of the sea this is everywhere the same. This difference in 

 density may have some effect in weakening the sun's rays, but probably 

 not so much as the circumstance of the atmosphere having a much 

 greater capacity for moisture at the equator than at the poles. At the 

 equator more moisture is required to produce saturation, and at the 

 same time the pocess of evaporation ia so powerful that the atmos- 

 phere is maintained on an average much nearer the point of saturation 

 than it is at the poles. These circumstances doubtless contribute 

 greatly to temper the solar rays. 



Plants from a great elevation, from within the tropics as well as 

 elsewhere, appear to require less shade than those from the level of the 

 sea. The rays of the sun have been ascertained to be more powerful 

 at 4000 feet above the level of the sea on the mountains of Jamaica 

 than at Port Royal. Hence the potatoe, a native of the high table 



land of South America, will not thrive under glass unless placed very 

 near it ; and if placed at a distance from it at which the pine-apple, a 

 native of the same country, but near the shore, will grow robust, the 

 potatoe will become pale and languid. 



These facts are too important to be omitted in explaining the prin- 

 ciples by which the construction of hothouses ought to be regulated, 

 and it will be found that those houses are the most perfectly suited 

 to the cultivation of plants in which such principles have been most 

 considered. 



With regard to the means of supplying artificial heat, the old system 

 of using brick flues in now rapidly being superseded by that of hot 

 water. Brick flues occupy a large space ; and are also liable to crack 

 and emit sulphureous effluvia to such an extent that the crops of early 

 forcing fruits have often been entirely destroyed. These objections 

 do not apply to hot-water pipes. When once fitted up they require 

 no repairs for many years ; whereas the brick flues must be frequently 

 broken up in order to clear out the soot. By hot water the distribu- 

 tion of heat can also be better regulated, and the uniformity of tem- 

 perature better maintained than by any other known means. The 

 methods of heating by hot water are various. The oldest and perhaps 

 the best for small houses is extremely simple, consisting of a boiler, 

 and, at the further end of the house, a cistern on a level with the 

 boiler. A pipe proceeding from near the top of the boiler, and com- 

 municating with the cistern at the same level, conveys the heated 

 water slowly from the former to the latter. Another pipe, situated 

 lower than the preceding, conducts the colder and consequently denser 

 portion of the water from the cistern to the boiler. This is frequently 

 called the return-pipe. A circulation is thus established in consequence 

 of the hottest and therefore the lightest portion of the water ascending, 

 and displacing the colder in the upper pipe, which from its greater 

 density tends to subside at the lowest level, which is in the lower or 

 return pipe. Here it would remain stationary, but the pipe com- 

 municating with the water in the boiler, a few inches above the bottom 

 of the latter, and the rarefied water in the boiler being unequal to tlu> 

 balancing of the colder and consequently denser portion in the return- 

 pipe, a continual ingress from the latter takes place into the boiler. 



This extremely simple form of the hot-water system has received 

 various modifications. Circulation has been obtained on the siphon 

 principle, the pipes being elevated above the level of the boiler, and a 

 vacuum being formed in them by pumping out or otherwise displacing 

 the air, which is replaced by the water, so that a greater descent is 

 afforded for the water in the return-pipe. Instead of large pipes, of about 

 four inches in diameter, a number of small pipes hermetically closed 

 have also been employed, and coils of such pipes have been inclosed in 

 a furnace instead of a boiler ; a large extent of radiating surface is thus 

 made to inclose a comparatively small quantity of water. As the water 

 becomes heated, a degree of pressure corresponding with its expansion 

 takes place ; this, however, requires to be regulated by an expansion- 

 tube to prevent explosion. Steam forced through pipes has also been 

 extensively used as a heating agent. Such modes undoubtedly afford 

 the means of raising the temperature more rapidly, and their appli- 

 cation in some cases may be attended with advantage ; but as the 

 cooling, if the fuel be not supplied regularly, takes place in the same 

 ratio of rapidity, it becomes a question whether a mode that produces 

 a slow and lasting heat, or one that is rapid in its production and 

 decline, is to be preferred. In order to solve this, it becomes necessary 

 to take into consideration the progressive amount of heat which is 

 required in hothouses relative to time. At noon, or soon after, the 

 natural temperature of this climate is generally at the highest, and the 

 temperature of the hothouse should also then be higher than at any 

 other time of the day or night. The external temperature declines 

 gradually till three or four A.M., when it reaches its minimum, of 

 perhaps 50 below the noon temperature. The hothouse temperature 

 should in like manner gradually decline, but only to the extent of 

 between 5 and 10. It is therefore evident that artificial heat must 

 be increased, if well applied, so as exactly to make up the deficiency ; 

 and it should by no means be allowed to have any more than one rise 

 and fall in the course of twenty-four hours. By a rapidly heating 

 apparatus, this is next to impossible ; and independently of all other 

 inconveniences, sudden changes of temperature are sure to affect the 

 hygrometrical state of the air in the house, causing condensation and 

 consequent dryness. A slow but effective and lasting heating power is 

 therefore preferable. 



Only a very few modifications of the plan of heating by hot water 

 require to be adverted to in this place. Instead of the pipes running 

 quite horizontally from the boiler to the cistern, they may be slightly 

 elevated towards the latter, which has the eBect of conveying a greater 

 proportion of the heat to the part of the house which is most remote 

 from the boiler. The boiler must be so constructed and placed as to 

 admit of the water contained in it being a little higher than the most 

 elevated portion of the upper pipe. Flat pipes are sometimes employed 

 as upper ones. As their transverse section is a parallelogram, and as a 

 circle contains more space than any other figure of equal perimeter, 

 these flat pipes contain less water in proportion to their surface, and 

 consequently the water in them is sooner heated to its maximum ; but 

 this, as previously shown, is no advantage, if in fact it be not a disad- 

 vantage, since it cools so much sooner. 



With regard to a supply of moisture, the above hot-water system, in 



