328 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



"The well-known law of pneumatics is simply this: If you take a ves- 

 sel holding one cubic foot of air, and with sufficient pressure you diminish 

 the volume of air to one-half a cubic foot, you have two quantities of air 

 in one space; or, as it is usually expressed, you have a pressure of two 

 atmospheres, if you take tha mercury colinrm in a barometer as the measure 

 of pressure. The atmosphere supports a column thirty inches high, a)id two 

 atmospheres occupying one space will support a column of mercury sixty 

 inches high, and so on for three, four, or more quantities. Hence the axiom, 

 'double the pressure is half the volume;' but, should the air be quickly 

 compressed, there would be an increase of temperature from the compres- 

 sion of the heat (or molecular action) contained in one volume of air, and, 

 consequently, that would be a little more than one-half the volume for 

 double the pressure until the temperature was tlie same as the original vol- 

 ume that was compressed. Now take this quantity of compressed air and 

 suddenly remove the pressure, and it would not quite be double the volume; 

 but, after the temperature had been acquired of the original quantity, it 

 would be exactly double the volume. 



"The foregoing statement is the complete definition of the much-talked- 

 of 'Mariotte law.' The only plausible way that this law can be applied to 

 steam is as follows: One cubic inch of water will make one cubic foot of 

 steam at the pressure of one atmosphere, or it will make one-half a cubic 

 foot of steam at the pressui'e of two atmospheres— and so on, by doubling 

 the pressure it will make half the volume nearly, 



"The actual proportions of volume and pressure, according to the tables 

 published by Pambour, Lardner, Brande, and others, are one cubic inch of 

 water at one atmosphere pressure makes 1,669 cubic inches steam ; two 

 atmosphere pressures make 881 cubic inches steam ; four atmosphere 

 pressures make 461 cubic inches steam ; eight atmosphere pressures n:»ake 

 249 cubic inches of steam. Whereas, if the Mariotte law perfectly applied 

 to steam the volumes would bo for one cubic inch of water at one atmo- 

 sphere 1,669 cubic inches steam; two atmosphere 834.5 cubic inches steam; 

 four atmosphere 417.25 cubic inches steam; eight atmosphere 208.125 cultic 

 inches st«am. So that 417 cubic inches steam at four atmosphere pressures 

 does not have water enough to make 1,669 cubic inches of steam at one 

 atmosphere by ]2| per cent., nor 208 cubic inches steam at eight atmo- 

 sphere by nearly 25 per cent. 



" That 457 cubic inches steam, at four atmosphere pressures would, on 

 gradually removing the pressure to one atmosphere, enlarge itself to 1,669 

 cubic inches, had not, as far as it was possible to learn, been determined 

 experimentally up to the year 1860. During that year it was tried in an 

 apparatus suggested by myself, the tables of which I may furnish in a 

 future paper." 



After quoting from Bourne's Treatise on the Steam Engine, on the appli- 

 cation of this law to the use of steam expansively, and the statements of 

 Eegnault of the motive power of elastic vapors, Dr. Kowell continued: 



"Herein is the difference between air and steam: If a cubic foot of air 

 at two atmospheres pressure, be contained in a tight vessel for a thousand 

 years, it will give out its elastic force on removing the pressure, while a 

 cubic foot of steam must give out its force in a few seconds, or else its 



