170 Mr. W. S. Day on Rowland's Value of 



constructed for the purpose. Rowland's thermometers were 

 originally compared with the air-thermometer in a vertical 

 position, and they were used in the same position in the 

 calorimeter. This made it seem more natural to make the 

 comparisons described here vertically also, but a number of 

 reasons of a practical nature made the author decide to make 

 them horizontally. The chief reason for this was the neces- 

 sity of taking the zero-points of the Tonnelot thermometers 

 after each reading, this being the method used at the Inter- 

 national Bureau, and which experience has shown to be the 

 only accurate one. It was necessary, therefore, to apply a 

 pressure-correction to the readings on Rowland's thermo- 

 meters to reduce them to what they would have been if the 

 comparison had been made vertically, but this was measured 

 very accurately by an apparatus similar to that used at the 

 International Bureau. In all other respects the attempt was 

 made to use Rowland's thermometers in the way in which he 

 used them in his experiments. 



The Tonnelot thermometers had their zeros determined 

 immediately after a measurement at any given temperature. 

 The zero-points were determined by plunging the thermo- 

 meters into a vessel filled with finely-crushed ice mixed with 

 distilled water. The ice used was very pure artificial ice. 

 The vessel in which the zero-points were taken was con- 

 structed so that the thermometers could be immersed in the 

 mixed ice and water to a distance of 7 centim. above the zero- 

 point. They were then read through a small ebonite tube 

 that extended horizontally through the tank from one side to 

 the other, and through which, at its middle point, the thermo- 

 meter stem passed. By a simple device the water in the 

 tank was prevented from running out. The object of this 

 arrangement was to make sure that all parts of the stem in 

 which the mercury was, as well as the bulb, should be at a 

 temperature of 0° C, and to avoid the presence of dew and 

 of drops of water on the stem, which, in the usual way of 

 taking zero-points, often interfere with the proper reading. 

 The thermometers were always read, in taking zeros, and in 

 the comparison-tank, by means of a micrometer, consisting 

 of a reading-telescope supplied with a micrometer-screw that 

 moved the whole telescope. 



From the compai*isons made, corrections were obtained for 

 each of Rowland's thermometers, which, when applied to 

 their indications reduced to the absolute scale by the tables 

 given in his paper on the mechanical equivalent, would make 

 them agree with the Paris hydrogen scale. From these cor- 

 rections Rowland's value of the mechanical equivalent was 



