THE MAGAZINE 



OF 



HORTICULTURE- 



SEPTEMBER, 1850. 



ORIGINAL COMMUNICATIONS. 



Art. 1. Polmaise Method of Heating GreenJiouses and 

 Hothouses, compared ivith Hotwater, scientifically and 

 practically considered. By R. B. Leuchars. 



In endeavoring to draw a comparison between the two 

 methods of heating above mentioned, it will be necessary 

 to consider briefly the conditions which constitute the pri- 

 mary laws of heat, and to which all methods of artificial 

 heating are subject. 



Heated bodies give off their caloric by two distinct modes, 

 radiation and conduction ; these are governed by different 

 laws, but the rate of cooling by both modes increases con- 

 siderably in proportion as the heated body is of greater or 

 less temperature above the surrounding medium. This 

 variation was long supposed to be exactly proportional to 

 the simple ratio of the excess of heat, that is to say, suppos- 

 ing any given quantity of heat evolved in a certain time, at 

 a specified difference of temperature, at double the difference,, 

 twice the quantity of heat would be given off in the same- 

 time. This law was originally proposed by Newton in his- 

 Principia, and although rejected as erroneous by some philoso- 

 phers it was adopted by many others, and was usually con- 

 sidered accurate until the elaborate experiments of Petit and 

 Dulong proved that, though approximately correct for low 

 temperatures, it becomes exceedingly inaccurate at the higher 

 degrees of heat. 



The cooling of any heated body, of whatever material, is 



VOL. XVI. ^NO. IX. 49 



