SECT, ii.] PROPERTIES OF STEAM. 65 



barometer tube, made perfectly dry, and filled it with mercury just boiled, marking 

 the place where it was stationary ; then graduated the tube into inches and tenths 

 by means of a file : into this tube he poured a little water, (or any other liquid the 

 subject of experiment,) so as to moisten the whole inside ; after this, he again 

 poured in mercury, carefully inverting the tube to exclude all air. The baro- 

 meter, by standing, sometimes exhibited a portion of water, &c. of one eighth or 

 one tenth of an inch, upon the top of the mercurial column, because being lighter 

 it ascends by the side of the tube ; which may now be inclined, and the mercury 

 will rise to the top, manifesting a perfect vacuum from air. He then took a 

 cylindrical glass tube, open at both ends, of 2 inches diameter, and 14 inches in 

 length ; to each end of which a cork was adapted, perforated in the middle so as to 

 admit the barometer tube, to be pushed through and to be held fast by them : the 

 upper cork was fixed two or three inches below the top of the tube, and half cut 

 away so as to admit water, &c. to pass by ; its service being merely to keep the 

 tube steady. Things being thus circumstanced, water of any temperature may be 

 poured into the wide tube, and made to surround the upper part of the vacuum of 

 the barometer ; and the effect of temperature in the production of vapour within can 

 be observed from the depression of the mercurial column. In this way, he says, he 

 had water as high as 155 surrounding the vacuum ; but as the high temperature 

 might endanger a glass apparatus, instead of it he used the following one for higher 

 temperatures. 



Having procured a tin tube of four inches in diameter, and two feet long, with a 

 circular plate of the same soldered to one end, having a round tube in the centre, 

 like the tube of a reflecting telescope ; he got another smaller tube of the same 

 length soldered into the larger, so as to be in the axis or centre of it : the small 

 tube was open at both ends ; and on this construction, water could be poured into 

 the larger vessel to fill it, whilst the central tube was exposed to its temperature. 

 Into this central tube he could insert the upper half of a syphon barometer, and fix 

 it by a cork, the top of the narrow tube also being corked : thus the effect of any 

 temperature under 212 could be ascertained, the depression of the mercurial 

 column being known by the ascent in the exterior leg of the syphon. Mr. Dalton 

 also remarks, that the force of vapour from water between 80 and 212 may be 

 determined by means of an air pump ; and the results exactly agree with those 

 determined as above. Take a florence flask half filled with hot water, into which 

 insert the bulb of a thermometer ; then cover the whole with a receiver on one of 

 the pump plates, and place a barometer gauge on the other : the air being slowly 

 exhausted, mark both the thermometer and barometer at the moment ebullition 

 commences, and the height of the barometer gauge will denote the force of vapour 

 from water of the observed temperature. This method may also be used for other 



