WILLIAM SIEMENS, F.R.S. 2 1 



rectly estimated, by a comparison of the two series of observa- 

 tions. 



Tin; second portion of this paper relates to the rate of expansion 

 of isolated steam by heat, that is, steam isolated from the water 

 from which it is generated. 



The author has not been able to meet with any direct experi- 

 ments on this subject, except some at a recent period by Mr. Frost, 

 cl 1 America, which, however, do not seem entitled to much con- 

 fidence. The rate of expansion of air and other permanent gases 

 by heat was first determined by Dalton and Gay Lussac simul- 

 taneously, who determined that all gases expanded uniformly, and 

 at the same absolute rate, amounting to an increase of bulk equal 

 to 4-J-oth P art f the total bulk at 32 Fahr. for every one degree 

 Fahr., or -o^th part of the total bulk at 212. Dulong and Petit 

 confirmed the law of Dalton and Gay-Lussac, but it appears that 

 these philosophers confined their labours to the permanent gases 

 and atmospheric pressure, and merely supposed the general appli- 

 cability of their discovery. 



Being interested in the application of " super-heated " steam, 

 the author tried some direct experiments on its rate of expansion, 

 in the year 1847, which confirmed his view, that vapours expand 

 more rapidly than permanent gases, or in other words, tliat the rate 

 of expansion of different gases and vapours is equal, not at the same 

 absolute temperature, but at points equally removed from their 

 points of generation, 



The apparatus employed in these experiments has been placed 

 before the meeting, and its simplicity, when seen in operation, is 

 such that the result, it is hoped, can hardly be doubted. 



It is shown in Fig. 2, Plate 3, and consists of a metallic trough 

 A A, containing oil, which is placed upon a furnace B B, heated by 

 gas flames. One end of the trough is provided with a stuffing- 

 box, through which a glass tube C, of about T Vth inch diameter, 

 and sealed at one end, may be slipped, which will rest horizontally 

 upon a scale below the surface of the oil. The mouth of the 

 glass tube is connected to an open mercury syphon G, with 

 either the one or the other leg filled with mercury, to produce the 

 desired pressure within the horizontal glass tube. A small drop 

 of water and a piston of mercury P being introduced into the 

 bottom of the tube, it is placed in the oil bath, and connected 



