94 



PRINCIPAL FORBES ON AN EXPERIMENTAL INQUIRY INTO 



civ 

 TABLE VII. — Showing the " Rate op Cooling,"—^- for Different Excesses of 



at 



Temperature (y). 





Case I. 



Case II. 





Case III. 





V. 



1J inch Bar, 



1J inch Bar, 



Ratio to I. 



1 inch Bar, 



Ratio to I. 





naked. 



papered. 





naked. 





1 



0009 



•017 



1 



00115 



- 



2 



•019 



•035 



► 174 



•0245 





3 



031 



•054 



0395 



1-28 



4 



•043 



•075 





056 





5 



•057 



•096 



> 



•072 



, 



10 



0124 



•203 



1-64 



•158 



1-27 



20 



•275 



•44 



1-60 



•34 



1-24 



30 



•43 



•72 



1-67 



•55 



1-28 



40 



•60 



1-01 



1-68 



78 



1-30 



50 



•80 



1-30 



1 62 



101 



1-26 



60 



101 



1-62 



1-60 



1-25 



124 



70 



1-21 



1-95 



1-61 



1-52 



1-26 



80 



1-42 



227 



1-60 



177 



1-25 



90 



L63 



260 



1-59 



2-04 



1'25 



100 



1-84 



2-95 



1-60 



233 



1-27 



120 



2-27 



367 



1-62 



2-92 



1-28 



140 



2-80 



4-40 



1-57 



3-50 



125 



160 



318 



508 



1-60 



4-03 



1-27 



180 



348 



5-75 



165 



4-50 



1-29 



200 



3-78 



*6-38 



1-69 



4-95 



1 31 



220 



*4-04 



*7-00 



173 



5-40 



1-34 



240 



*429 



*765 



178 



*5-85 



1-36 



260 



*4-52 



*8-28 



1-83 



*6-30 



1-39 



280 



*4-75 



*8-90 



1-88 



*672 



1-42 



The num 



bers marked thus * being the results of calculation, are to be regarded as 



more or less h 



ypothetical, and increasingly so at the higher temperatures. 



90. Sixthly, I will not attempt to account for the inflections of the curves of 

 Plate V. on physical principles, farther than to remark that the rapid increase in 

 the velocity of cooling with temperature in the lowest part of the scale is perhaps 

 owing to the separate effects of cooling by radiation, and cooling by convection. 

 It seems probable that a certain excess of temperature of the bar above the air is 

 necessary to determine efficient atmospheric currents, and thus to accelerate the 

 rate of cooling ; that, in fact, there is an amount of viscosity in air, which it re- 

 quires a certain elevation of temperature properly to overcome. I would also 

 observe, that the cooling in Case I. is (at higher temperatures) less regular than 

 in the two other cases, while in Case III. the logarithmic law is almost accurately 

 observed at those temperatures. This is no doubt to be ascribed to the greater 

 mass of the Bar No. L, compared to its radiating power, occasioning probably 



