P0LMA1SE METHOD OF HEATING HOT-HOUSES. 



67 



powerful character, being employed. All the merits of my 

 stove, whatever these may be, are due to Dr. Arnott ; but I trust 

 that the great principle of Polmaise, the conditions under which 

 the stove is placed, will never be lost sight of ; for on no other 

 principle can atmospheric heat be so naturally, so cheaply, or so 

 uniformly diffused. 



During the last two months I have had repeated opportunities 

 of making thermometric experiments in the hot-house at Nut- 

 field, and also other trials of a very interesting character, and 

 leading to very important practical conclusions. The hot air, 

 as it enters the hot-house from the chamber, passes beneath the 

 bottom of the plunging bed, exactly at its point of entrance ; one 

 of the slates on which the bed rests has been removed, leaving 

 an area of about 2 feet, or half the hot-air opening ; and by 

 this means a thermometer can be suspended in the hot blast 

 at its entrance. I have seen this indicate 174° Fahr., but it 

 commonly ranges from 120° to 150°. When about the latter 

 point, another thermometer suspended one foot above the former 

 will indicate from 85° to 90°, ; while a third, on the same level 

 with the second, but at one foot horizontal distance, will indi- 

 cate only 65° ; and other thermometers hanging in different parts 

 of the house, at one end or both, or in the centre, indicate 63° ; 

 while even a thermometer on the floor, at the extreme end of the 

 house, will indicate 61°. The only unequal portion extends about 

 2 feet around the hot-air opening, and in the ridge of the house, 

 where we must always expect to find the temperature several 

 degrees higher. The temperature of the plunging bed varies 

 from 80° to 92° Fahr. in different portions, which is accounted for 

 by the fact, that much of the hot air escapes through the open 

 ventilators before reaching the extreme end of the bed ; but for 

 many purposes this variation is desirable, and all this may be 

 regulated, according to circumstances, by the side ventilators. 

 The temperature of the plunging bed is peculiarly steady, arising 

 from the material being a bad conductor of heat, slow to heat 

 and slow to cool. The inference to be drawn from all the expe- 

 riments I have made is, that there will be no occasion to provide 

 any means of distributing the hot air; it will equally diffuse 

 itself. Another point I have determined is, that the lower the 

 external temperature, the greater the proportionate difference 

 between the hot-house and the external air, arising partly from 

 the principle of compensation already noticed. Thus the ave- 

 rage temperature of the hot-house above the external air, with 

 moderate fires at night, is about 20° ; but on the only occa- 

 sion in which I have yet had an opportunity of testing it in 

 frost, the house at night was 30° above the external air, and 

 never during the night was the difference more than 26°. It 



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