526 



THE FARMERS' REGISTER. 



may reduce it very much below the expectations 

 of the cultivator. 



The most unsuspected of these is the desrruc- 

 tionoC aqueous vapor, by the hot, dry, absorbent 

 sur(ace of flues. The advantages derived Crorri 

 hot-water pipes, or sleam pipes over brick flues, 

 are so well known, as not to require any evidence 

 to prove the fact. Gardeners explain the difler- 

 ence in the action of the two, by saying that the 

 dry heat produced by hot-water pines is siceeier 

 than that given off' by flues ; which is not a very 

 intelligible expression. The (act is, that the air 

 of houses heated by flues is under equal circum- 

 etancee, much drier than that where hot- water 

 pipes are employed ; because the soft burnt clay 

 of the brick flues robs the air of its moisture, 

 while theunabsorbent surlace of iron pipes ab- 

 stracts nothing. 



Another source of dryness is the coldness of the 

 glass roof, especially in cold weather when its 

 temperature is lowered by the external air, in con- 

 sequence of which the moisture of the artificial 

 atmosphere is precipitated upon the inside of the 

 glass, whence it runs down in the form of " drip." 

 Mr. Daniell observes that the glass of a hot- 

 house, at nijJht, cannot exceed the mean of the 

 external and^ internal air; and, taking them at 

 80° and 40°, 20 degrees of dryness are kept up 

 in the interior, or a degree of saturation not ex- 

 ceeding .528. To this, in a clear night, we may 

 add a? least 6° for the eflecis of radiation, to 

 which the glass is particularly exposed, which 

 will reduce the saturation to .424 ; and this is a 

 degree of drought which must be very prejudicial. 

 It will be allowed that this is not an extreme case, 

 and much more favorable than must frequently 

 occur during the winter season. Some idea, he 

 adds, may be Ibrmed of the prodigiously increas- 

 ed drain upon the functions of a plant, arising 

 from an increase of dryness in the air, from the 

 following consideration :— If we suppose the 

 amount of its perspiration, in a given time, to be 

 57 grains, the temperature of the air being 75° 

 and^'the dew-point 70«, or the saiuration of the 

 air being .849, the amount would be increased to 

 120 grains in the same time, if the dew-point 

 were'^to remain stationary, and the temperature 

 were to rise to 80° ; or, in other words, if the 

 saturation of the air were to lall to .726 (Hort. 

 Trans., vi. 20.) It is well known that the effect 

 of maintaining a very high temperature in hot- 

 houses at night, during winter, is frequenilyto 

 cause the leaves to wither and turn brown, as if 

 scorched or burnt , and this is apparently ovying 

 to the dryness of the air, in consequence of the 

 above causes. 



It is evident that the mode of preventing this 

 drying of the air by the cold surface of a glass 

 roof will be either by raising the temperature of 

 the glass, which can only be effected by drawing 

 a covering of some kind over our houses at night, 

 so as to mtercept radiation, or by double glass 

 sashes ; or else by keeping the temperature of the 

 air of the house as low as possible, consistetiily 

 with the safety of the plants, and so diminishing 

 ttie difference between the temperature of the ex- 

 ternal and internal air. 



A bad system of ventilation is another cause of 

 the loss of vapor in "the atmosphere of gla/ed 

 houses, to which reference will be made in the 

 succeeding chapter. 



It is, in all appearance, fo the attention that, 

 since the appearance of Mr. Danieli's paper, in 

 1824, upon this subject, has been paid to the at- 

 mospherical moisture of glazed houses, that the 

 great superiority of modern gardeners over those 

 of the last generation is mainly to be ascribed ; 

 there are, however, traces of the practice at a 

 much earlier period, although, from not under- 

 standing the theory of the practice, no general 

 improvement took place. In the year 1816, an 

 account was laid belbre the Horticultural Society 

 of a very successful mode of forcing grapes and 

 nectarines, as practised by Mr. French, an Essex 

 farmer, with very rude materials, and under unfa- 

 vorable circumstances. It is not a little remarka- 

 ble, that, alihough Mr. French himself correctly 

 referred his success to the skilful management of 

 the atmospherical moisture of his forcing-houses, 

 the subject was so little understood at that time, 

 that the author of the account not only shrank 

 from adopting the opinion, but evidently, from the 

 manner in which he speaks of Mr. French's ex- 

 planation, had no idea ofits justness. The method 

 itself is sufficiently remarkable to deserve being 

 extracted. 



" About the beginning of March, Mr. French 

 commences his forcing, by introducing a quantity 

 of new long dung, taken from under the cow-cribs 

 in his straw yard ; being principally, if not entire- 

 ly, cow dung ; which is laid upon the floor of his 

 house, extending entirely li*om end to end, and 

 in width about six or seven feet, leaving only 

 a pathway beliween it and the back wall of the J 

 house. The dung being all new at the begin- Ij 

 ning, a profuse sieam arises with the first heat, 

 which, in this stage of the process, is found to be 

 beneficial in destroying the ova of insects, as 

 well as transfusing a wholesome moisture over 

 the yet leafless branches ; but which would prove 

 injurious, if permitted to rise in so great a quantity 

 when the leaves have pushed Ibrih. in a few 

 days, the violence of the steam abates as the 

 buds open, and in the course of a fortnight the 

 heat begins to diminish ; it then, becomes neces- 

 sary to carry in a small addition of fresh dung, 

 laying it in the bottom, and covering it over with • 

 the old dung fresh forked up: this produces a 

 renovated heat, and a moderate exhalation of 

 moist vapor. In this manner the heat is kept 

 up throughout the season, the fresh supply of 

 dung being constantly laid at the bottom in order 

 to smother the steam, or rather to moderate the 

 quantity of exhalation ; for it must always be 

 remembered, that Mr. French attaches great 

 virtue to the supply of a reasonable portion of" the 

 vapor. The quantity of new dung to be intro- 

 duced at each turning must be regulated by the 

 greater or smaller degree of heat that is found 

 in the house, as the season or other circumstances 

 appear to require it. The temperature kept up 

 is pretty regular, being from 65 to 70 degrees." 

 (Hort. Trans., i. 245.) 



In this case, which attracted much attention at 

 the time, it is evident that the success of the prac- 

 tice arose principally out of two circumstances: 

 firstly, the moisture of the atmosphere was skil- 

 fully maintained in due proportion to the tempera- 

 ture ; and, secondly, a suitable amount of bottom 

 heat was secured. This is, as will be elsewhere 

 remarked, the principal cause of the advantages 

 tbund to attend the Dutch mode of forcing. The 



