the sudden Expansion of compressed elastic Fluids. 253 



gree near the point where the air or the steam is released 

 from compression. (See figs. 2. 3. & 4.) 



2ndlv, After the air has been suddenly expanded, it quickly 

 recovers a large portion of its former density. (See figs. 3. & 4.) 

 3rdly, A portion of elastic fluid of a given density is dis- 

 placed by another portion of the same fluid, of much less den- 

 sity than that of the fluid which has been displaced. (See figs. 

 3. & 4.) 



In October 1822, I read a paper before this Society, giving 

 an account of some experiments which I had then made on 

 this subject, and suggesting an explanation of the results on 

 mechanical principles. I did not offer it then, nor do I offer 

 it now, with the expectation that it will be generally con- 

 sidered as satisfactory. All the experiments, however, which 

 I have made since that time, admit, as it appears to me, of that 

 explanation ; and I have had the satisfaction to find that it has 

 been adopted by some very good experimenters. 



Such as it is,' I beg leave to offer it again, after an interval 

 of six years, to the consideration of this Society. 



If we suppose A and B (fig. 5.) to be two equal balls of 

 lead attached to the 



opposite ends of an Fig. 5. 



elastic spring; and 

 if we suppose the 

 spring, in the posi- 

 tion of A and 13, to 

 be in a neutral state, 

 that is, to have no 

 tendency either to 

 expand or collapse. 

 If we next sup- 

 pose the balls to 



be compressed together as at C and D, and then suddenly 

 set at liberty, they will, by the joint action of elasticity and 

 momentum, spring out, not only to the distance they were at 

 originally as at A and B, but to a distance as at E and F, as 

 much beyond their original distan'^e as they had been com- 

 pressed within it. After having been separated to that di- 

 stance, they will collapse and vibrate to and fro for a time. 



Now if elastic fluids, such as air and steam, consist of se- 

 parate particles, we know that these particles are kept asun- 

 der by an elastic force, the same in effect as if steel springs 

 were interposed between them ; and we cannot doubt that 

 such particles have momentum like all other ponderable mat- 

 ter ; neither can I see any reason to doubt that tiie suilden 



expansion 



