252 Mr. Ewart on the Phisnometia attending 



the experiments (fig. 4.), in November 1828, and he was satis- 

 fied that the results were correctly stated. 



Various explanations have been proposed of some of the. 

 phaenomena which I have described. 



It has been supposed, that the ascent of the mercury in 

 figs. 2. & 3, and of the coloured water in fig. 4, is occasioned, 

 not by the rai-efaction of the fluid in contact with the upper 

 ends of these tubes, but by the particles of fluid in the 

 tubes (whether these particles be of air, water, or mercury) 

 being drawn or sucked out by some kind of lateral action of 

 the issuing fluid. But if there were any action of that kind, 

 its effects would have been apparent in the inverted syphon 

 (fig. 3.); and the mercury in the leg next to A (in which the 

 air was moving at the rate of forty-five feet per second) would 

 have been elevated instead of being depressed. 



I applied an inverted syphon to the air-reservoir of a similar 

 blowing apparatus, in which the interior pressure was equal 

 to 33*5 inches of mercury (including the atmosphere), while an- 

 other syphon was applied to the conducting pipe, as in fig. 3, 

 at the distance of twelve feet from the reservoir. The air was 

 passing along the pipe with the velocity of forty-eight feet per 

 second, and the interior pressure was only l-268th part less 

 in the conducting-pipe than in the reservoir. 



Explanations of the low temperature of high-pressure steam, 

 at the place where it issues, have indeed been proposed, with- 

 out any reference to the rarefaction of the steam at that place. 

 It has been held l)y some, that the steam issues with so great 

 a velocity at that place, that it has not time to give out its 

 heat; that unless part of the steam be condensed into water, 

 little or none of its heat can be given out ; and it has been 

 asserted that when the hand is presented to such steam, it re- 

 mains dry. Others have held, that the current of steam car- 

 ries with it a current of air, attracted in some manner to its 

 sides, by which the bulb of the thermometer is cooled. — My 

 hand, however, has always been wet by the steam when pre- 

 sented to it ; and in most of the experiments which I have made, 

 the bulb ot the thermometer was surrounded with steam ; so 

 that it could not be affected by any external air supposed to 

 adhere to the sides of the current of steam. 



That the temperature of high-pressure steam, on being re- 

 leased, should come down to the temperature of steam of at- 

 mospheric pressure, is what might be expected. But how it 

 instantly falls so much below 212° requires some explanation. 



There are three circumstances to be observed in the fore- 

 going experiments, which appear to require particular attention. 



1st, The reduced pressure takes place in the greatest de- 

 gree 



