164 ANNUAL OP SCIENTIFIC DISCOVERT. 



the steam entering this space would rise much higher than that of the steam 

 in the boiler. Or, suppose two vessels side by side, one of them vacuous 

 and the other filled with air at, say, two atmospheres, a communication being 

 opened between the vessels, the prcssiire would become equal in the two ves- 

 sels; but the temperature would fall in one vessel and rise in the other; and 

 although the air is expanded in this manner to a double volume, there would 

 not on the whole be any appreciable loss of heat ; for, if the separate por- 

 tions of air be mixed together, the resulting average temperature of the 

 whole would be very nearly the same as at first. It has been proved experi- 

 mentally, corroborative of this argument, that the quantity of heat required 

 to raise the tempei-ature of a given weight of air to a given extent, was the 

 same, irrespective of the density or volume of the air. Regnault and Joule 

 found that to raise the temperature of a pound weight of air one cubic foot 

 in volume, or ten cubic feet, the same quantity of heat was expended. 



In rising against the force of gravity steam becomes colder, and partially 

 condenses while ascending, in the effort of overcoming the resistance of 

 gravity, by the conversion of heat into water. For instance, a column of 

 steam Aveighing, on a square inch of base, 250.3 pounds, that is, a pressure 

 of 2-50.3 pounds per square inch, would, at a height of 275,000 feet, be re- 

 duced to a pressure of one pound per square inch ; and ascending to this 

 height, the temperature would fall from four hundred and one degrees to one 

 hundred and two degrees Fahrenheit; while, at the same time, nearly twenty- 

 five per cent of the whole vapor would be precipitated in the form of water, 

 if not supplied with heat while ascending. 



If a body of compressed air be allowed to rush freely into the atmosphere, 

 the temperature falls in the rapid part of the current by the conversion of 

 heat into motion; but the heat is almost all reproduced when the motion is 

 quite subsided; and from recent experiments it appears that nearly similar 

 results are obtained from the emission of steam under pressure. 



When water falls through a gaseous atmosphere its motion is constantly 

 retarded as it is brought into collision with the particles of that atmosphere, 

 and by this collision it is partly heated and partly converted into vapor. 



If a body of water descends freely through a height of seven hundred and 

 seventy-two feet, it acquires from gravity a velocity of two hundred and 

 twenty-three feet per second; and if suddenly brought to rest when moving 

 with this velocity, it would be violently agitated, and raised one degree in 

 temperature. But suppose a water-wheel, seven hundred and seventy-two 

 feet in diameter, into the buckets of which water is quietly dropped, when 

 the water descends to the foot of the fall, and is delivered gently into the 

 tail-race, it is not sensibly heated. The greatest amount of work it is possi- 

 ble to obtain from water falling from one level to another lower level is 

 expressible by the weight of water multiplied by the height of the fall. 



The object of these illustrative exhibitions of the nature and reciprocal 

 action of heat and motive power, with their relations, are : first, to familiarize 

 the reader with the doctrine of the mechanical equivalence of heat; second, 

 to show that the nature and extent of the change of temperature of a gas, 

 while expanding, depends nearly altogether upon the circumstances imder 

 which the change of volume takes place. 



NEW METALLIC THERMOMETERS. 



A new registering thermometer invented by Dr, James Lewis, of Mohawk, 

 N. Y., has the following construction: 



