160 



PRINCIPLES OF HEATING HOT-HOUSES. 



we shall find what is the amount of radiation under the two 

 different states of surface, noticed at the top of the second and 

 third columns. * 



Some very remarkable effects may be perceived by an inspec- 

 tion of the above table. It appears that the ratio of heat lost by 

 contact of the air alone, is constant at all temperatures ; that is, 

 whatever is the ratio between 40 and 80, for instance, is also 

 the ratio between 80 and 160, or between 100 and 200. 

 This law is expressed by this formula : 



where t represents the excess of temperature, and n a number 

 which varies with the size of the heated body. In the case 

 represented in the foregoing table, n = 0.00857. 



Another remarkable law, is that the cooling effect of the air is 

 the same, for the like excess of heat, on all bodies, without 

 regard to the particular state or nature of their surface. This 



* The temperatures of this table are expressed in degrees of the Cen- 

 tigrade thermometer, as the zero of this thermometer is the freezing 

 point of water, and from that to the boiling point of the same fluid is 

 100. In order to find the number of degrees on Fahrenheit's scale, 

 which answers to any given temperature of the Centigrade, multiply 

 the number of degrees of Centigrade by 9, and divide the product by 5 j 

 add 32 to the quotient thus obtained, and this sum will be the number 

 of degrees of Fahrenheit required. 



