534 MR J. Y. BUCHANAN ON 



the stem, the thermometer is preferably pulled up, so that the bulb is entirely in the 

 steam. The steam so produced in an apparatus of the proportions described, must be 

 truly saturated steam of the tension equal to the actual barometric pressure, and must, 

 therefore, have exactly the temperature which corresponds to this pressure. This is 

 the temperature of steam condensing on pure water. 



The supply of steam is abundant, and the latent heat of steam is very great ; the 

 thermometer, therefore, must take, in a very short time, exactly the temperature of the 

 steam with which it is completely surrounded. The thermometer may be constructed 

 so that a millimetre on the stem corresponds to a hundredth or a thousandth of a degree. 

 Its indication will be perfectly steady, provided that the conditions remain unchanged. 

 The slightest change in the barometric pressure makes itself at once apparent, and at 

 all times, especially during unsettled weather, the temperature of saturated steam must 

 be observed at frequent intervals. 



It is convenient to have a separate apparatus for this purpose. If we imagine the 

 receiver, C, with the entry tube at the bottom straight instead of bent, so that it can 

 take the place of the T tube in the boiler, we have a perfect apparatus for determining 

 the temperature of saturated steam and consequently also for fixing the point correspond- 

 ing to 100° C. on the scale of the thermometer. It is assumed that the steam tube 

 is large enough to take the whole working part of the thermometer. It is unsuitable 

 in itself or to the steam generator used if the steam makes its exit with an audible 

 sound. Tenths of a Centigrade degree are then uncertain. Yet it is essential that 

 there should be an abundant flow of steam through the tube ; therefore, the exit tube 

 must be sufficiently wide to allow the steam to issue in a stream of good volume, and 

 must not be so wide as to incur any risk of regurgitation of air. Again, the entry 

 tube for the steam must not be too narrow. It is not essential, but it is very con- 

 venient, that it should be so wide that the steam condensed on the walls and flowing 

 back into the boiler should continue to flow down the sides of the entry tube and 

 not collect at the bottom of the wide part of the tube. When the steam proceeds 

 along a short straight passage from the boiler to the steam tube, it throws about any 

 water in a violent and inconvenient way. In a tube which I use for this purpose the 

 entrance tube has a diameter of 9 mm. and the exit tube 10 mm. The entry tube is thus 

 wide enough to permit the condensed steam to flow back along its sides ; at the same 

 time, it is smaller than the exit tube, so that, apart from the continual condensation of 

 a portion of the steam, there is no danger of the tube receiving more steam than it 

 can freely get rid of. 



That an apparatus such as that here described does, in fact, exclude the possibility 

 of the steam supplied having a tension which differs at all from the pressure of the 

 atmosphere with which it exhausts, will be evident from the following experiments. 

 The small copper flask, the spirit lamp, and the steam tubes above described were 

 used. The thermometer was divided into fiftieths of a degree Centigrade, the length 

 of one degree being 35 mm. The atmospheric pressure happened to be pretty high, and 



