20 



HEAT. 



and therefore of invariable length, and in measuring the difference 

 between this length and the length of the scale whose expansion is 

 sought, the temperature of the second scale being varied. For the 

 comparison, two microscopes are placed vertically, as nearly as possible 

 one metre apart, passing through projections overhanging from the tops 

 of two massive pillars (Fig. 12), the bodies of the microscopes being very 

 firmly attached to the projections. Two parallel troughs, somewhat more 

 than a metre in length, are fixed to a table running to and fro on rails, 

 one of the scales being placed in each, so that either scale may be brought 

 with its end-marks under the microscopes. The troughs are double- 

 walled, and a stream of water, kept at a constant temperature by a 

 thermostat, circulates in the space between the walls in each. The inner 

 troughs are also filled with water, one being kept at a constant tempera- 

 ture, while the other is heated to successive higher temperatures. The 

 difference in the lengths of the two scales is measured at each tempera- 

 ture by the microscopes, which are provided with micrometer eyepieces 

 for the purpose. Special arrangements are adopted for the adjustment 

 of the scales in position, and stirrers in the form of turbines are used to 



FIG. 12. Diagram of Expansion Apparatus at the Bureau International 



mix up the water thoroughly before each reading is taken, so that the 

 temperature throughout the trough is uniform. The thermometers used 

 are all studied carefully, so that the temperature is ascertained with 

 great accuracy. Full details of the method are given in Travaux et 

 Memoires du Bureau International des Poids et Mesures, vol. ii. 



Another method of the same class was devised by Pouillet, to 

 measure expansions of bars at very high temperatures, as in a furnace. 

 The bar to be tested was placed horizontally in a chamber, the tempera- 

 ture of which could be regulated, and its two ends were viewed through 

 windows in the chamber by two horizontal telescopes of short focal 

 length. Arrangements were adopted so that all expansion took place at 

 one end and the rotation of the telescope to keep the end in the centre 

 of the field at the high temperature was observed. 



Method of Lavoisier and Laplace. Shortly before Ramsden 

 made his experiments, an apparatus was devised and used by Lavoisier 

 and Laplace, though their results were not published till many years 

 later. The arrangement will be understood by the aid of Fig. 13. 



The bar BB, of which the expansion was sought, was supported 

 horizontally on glass rollers rr in a trough filled with liquid. One end 

 abutted against a vertical, fixed, glass rod FF, suspended from a cross- 

 piece T supported by two firm pillars, of which only the back one is 



