RED-HOT IRON ESTIMATED IN FOOT-POUNDS. 73 



intensity of heat in the furnace flue, by the move-' 

 ment of the magnetic needle, which performed the 

 functions of a thermometer. 



This experiment illustrates, on a minute scale, 

 the frequent movements of compass needles, ob- 

 servable when sudden changes of temperature are 

 produced by sunshine and cold storms in adjacent 

 localities, and more especially while the flashes of 

 the aurora borealis occur. The eruption of red- 

 hot lava from volcanoes, and other causes, are 

 found to produce nearly simultaneous perturba- 

 tions of compass-needles all over the globe, as 

 verified in national magnetic observatories. 



The ready conversion of heat into electric cur- 

 rents, that transmit the excitation to remote dis- 

 tances over the earth's surface, accounts for the 

 sudden disappearance of heat on the condensa- 

 tion of steamy vapors of thunder-clouds, and vari- 

 ous phenomena of " latent heat." 



Taking for a basis of calculation the mechan- 

 ical action of seven hundred and seventy-two 

 foot-pounds as the equivalent of the molecular 

 vibration of one pound of water, in raising its tem- 

 perature one degree from 39 to 40 Fahrenheit, 

 and estimating the temperature of red-hot iron at 

 about 1000, and assuming the increment of heat 

 in iron to be the same as in water (772 x 1000 =), 

 77 2,000 foot-pounds may represent the intensity 

 of molecular vibration of a pound of red-hot iron ; 

 and may serve also to indicate the extent of me- 



