PROGEEDINGS OF THE POLYTECHNIC ASSOCIATION. 329 



force is entirely lost. Also the relative volume of air at different 

 pressures, of which the Mariotte law is the exponent, depends on the same 

 temperatur^; whereas the different pressures of steam depends wholly on 

 different temperatures ; for instance, steam at the pressure of ore atmo- 

 sphere is Sri*^ F.; two atmospheres, 250* F.; three atmospheres, 274'' F. 



" Now, the slightest increase of pressure at these temperatures, or 

 slightest decrease of temperature at these pressures, will turn the wliole of 

 the steam to water; while an increase of pressure on the air will only 

 diminish its volume to the amount due to that pressure. 



"Tlie modern received opinion promulgated by Joule, that heat is con- 

 verted into force in the steam engine, is in accordance with the statement 

 made by Regnault, tfiat the amount of power developed by the expansion 

 of any elastic fluid is always in proportion to the loss of heat undergone 

 by this fluid in the part of the machine where the power is produced. Tiie 

 quantity of heat, or, as it is expressed, the 'total heat,' as ascertained by 

 M. Kegnault from actual experiment, in a cubic inch of water in steam at 

 one atmosphere pressure is Ijl^S*^ F.; two atmospheres pressure, 1,190° F.; 

 four atmospherics pressure, 1,203" F.; eigtt atmospheres pressure, 1,218" F. 



" If the force is all a heat force, and it is properly applied in moving the 

 piston of a steam engine, and as it is not possible to increase this 'heat by 

 expanding the steam, it would seem as if some of the modern theorists are 

 endeavoring to make out that the steam can work three or four times over, 

 or, as some of the most enthusiastic saj-, ' expand a thousand times.' The 

 experinu-nts of Regnault, to determine the theoretical motive power of 

 expansion, being 'extremely delicate and difficult, are not applicable to so 

 rude a machine as a steam engine;' they of course furnish no rule to calcu- 

 late the motive power produced by expansion in a steam engine. 



" We are finally left to recent experiments on the steam engine itself, 

 and these, so far as they have seen them fairly tried, show that the ' noto- 

 rious' multiplying of its performances by expansion is founded upon 'hypo- 

 thesis' of no great probability." 



Tiie following interesting scientific intelligence was presented from the 

 Chair: 



Glasses for Viewing the Sun. 



Sir John Herschel, in a late number of the Quarterly Journal of Science, 

 says: "To use the full aperture of the telescope is of paramount neces- 

 sity, either in viewing the sun or the planets. If the extinction of light is 

 effected by colored glasses, the best combinations I yet have found are, first, 

 that of two plane glasses, of a shade between brown and violet, with one 

 of grass green hue interposed; or, second, that of two green glasses with 

 a blue one colored with cobalt between them. They allow scarcely any 

 rays of the spectrum to pass but the yellow and less refrangible green; and 

 they cut off about all the heat. The perfection of vision is obtained by 

 using only the extreme red rays; but glasses which transmit these cannot 

 be used, on account of the heat wiiich they allow to pass. Whatever com- 

 bination of glasses be used, they are, however, apt to crack and fly to 

 pieces, through the heat they intercept. 



