lietrosj)edive Criticism* 273 



these defects, and produces a uniformly saturated atmosphere which must be 

 wholesome alike to plants and men. 



There is a fact, which I have often observed in a small stove devoted to the 

 cultivation of Orchideae, which rather confirms this theory. This stove is fur- 

 nished with most abundant apphances for moistening the atmosphere; about 

 15 square feet of water surface to 80 sq. ft. of glass. Of this water surface, 

 8 ft. are always at from 100° to 145° ; the remainder varies from 80° to 85°, 

 being warmer than the house both by night and day : and all this is above 

 the level of the heating pipes. The atmosphere, therefore, is, I believe, 

 damper than that of almost any other orchidaceous house in England ; and 

 at this season the leaves of the plants are every morning covered with an 

 almost tropical dew, standing in large drops all over them. Now in this 

 stove, v.'ben the awning is on, and radiation from the glass, and consequently 

 deposition from the air, much impeded, a temperature of 80° is by no means 

 unpleasant. In five minutes after the awning is off, that same atmosphere 

 becomes most oppressive, I believe because it has lost a portion of the 

 water which it held in solution. — J. R. Sevenoahs, April 9. 1840. 



On Mr. Penn's Method of Fentilatioji, and Mr. Rogers's Conical Boilers. — 

 The discovery of the best method of heating buildings being a subject of 

 great importance in horticulture, I have perused with much attention and 

 interest the several articles in your valuable Magazine for March, and I beg to 

 offer the following remarks, as the conclusions of a practical man. 



I have always been of opinion that the healthiness of plants, and their 

 complete developement and perfection, are best secured by a moderate degree of 

 ventilation, in opposition to that plan which would assert perfection to consist 

 in keeping plants without any change of air whatever. I am, however, at a 

 loss to understand how this is accomplished by Mr. Penn's process. 



From your description, and the diagrams which are given, no change what- 

 ever can occur in Mr. Penn's arrangement, which merely provides for the 

 continual reheating of the same air over and over again. If this be the case 

 (for no mention is made of any method by which the foul air can escape), in 

 what does this plan differ from every other method of heating by hot water ? 

 In all buildings heated by hot-water pipes, there must, of necessity, be a 

 constant motion in the air ; for those particles of air which come in contact 

 with the pipes become expanded by the heat, and rise upwards, their place 

 being supplied by colder and more dense particles. A continual motion is 

 thus kept up in the whole atmosphere ; for such is the extreme mobility of 

 the particles of air, that a current, however small, established in any direction, 

 will draw into its vortex many thousand times its own bulk of the same 

 fluid. 



If, then, the plan affords no change of air, and only produces a circulation 

 among its own particles, similar to that effected by other arrangements, in 

 what, allow me again to ask, does its excellence consist ? From the enco- 

 miums passed upon it by you, I am disposed to think I misunderstand you; 

 and, if so, shall feel greatly obliged, in common, no doubt, with many others 

 of your readers, if you will set me right. 



But there are other reasons why I am disposed to doubt the superiority of 

 Mr. Penn's plan. By placing the pipes in a drain or tunnel, it is certain that 

 the heat cannot be so regular as when the pipes are distributed in the house 

 itself; nor can the same amount of surface produce the same temperature, as, 

 when they are exposed, the air begins to rise with the smallest possible increase 

 of heat. 



It is certain that less than one degree of heat will cause the air in contact 

 with the pipes to ascend ; but, when the heating surface is enclosed in a 

 drain, it requires a much greater force of heat to cause its ascension, on 

 account of the diverging currents which are produced, and the friction of the 

 air in passing through the various gratings, tunnels, and apertures. This is 

 exemplified in many cases where large buildings have been warmed by a hot- 

 water apparatus placed in the basement, and the heated air is brought into the 



