\(J) 



432 Effects of Percussion and Annealing on Steel Magnets. 



Mr. Bottomley's modification of the air-thermometer, re- 

 ferred to above, which was used for measuring the high tem- 

 peratures, is constructed and employed as follows : — 



Suppose a glass tube, -J inch or f inch internal diameter, 

 is made to the shape shown in fig. 1, which -p. , 



is a quarter of the full size of the tubes used 

 in these experiments. 



The parts AB and DC are drawn out to 

 fine capillary tubes, very small in volume in 

 comparison with the bulb BD of the ther- 

 mometer. When ready for use it is com- 

 pletely filled with pure dry air and closed at 

 C, but open at A. 



The parts CDB and the greater portion of 

 AB are now inserted into the liquid, the 

 temperature of which we wish to measure ; 

 and w r hen it has been in long enough to be 

 at the same temperature as the liquid, it is 

 sealed at A with a blowpipe flame, thus en- 

 closing a sample of the air at the required 

 temperature. The height of the barometer 

 at the time of closing is also noted. 



It is then taken out and allowed to cool, and also thoroughly 

 cleaned, with alcohol if the bath has been of oil, as it was in 

 the case under consideration. 



It is now carefully weighed in a chemical balance ; then 

 the end C is opened under water at a known temperature; the 

 height of the barometer being again noted. 



By this operation the water is allowed to rush into the bulb 

 BD and to compress the contained air to the volume consistent 

 with the barometric height and temperature at the given instant. 



The thermometer with the contained air and water is again 

 carefully weighed, at the same time taking care to add the 

 small piece of tube which was broken off in the act of open- 

 ing the end C. The remaining part of the tube AB is now 

 filled w r ith water by breaking off the end A, and the whole 

 again carefully weighed. 



In the following calculation the weight of the air displaced 

 during this last operation is assumed to be so very small that 

 for our present purpose we may neglect it. 



Let now 



g = Weight of the glass, in grammes. 

 g + Wl =z Weight of the glass and the contained air, in 



grammes. 

 g + w 2 = Weight of the glass and water, in grammes. 

 t = Temperature of the water employed. 



