312 Daniell o« a New Pyrometer. 



of heat causes the metal bar and wire to expand more than the 

 earthen tube; the consequence is, that ihe action of the spring 

 always keeping the wire tight draws the wheel round. In 

 cooling the metal again contracts, and restores the spring to its 

 former degree of tension. 



If instead of heating the instrument gradually, it be plunged 

 at once into a brisk fire up to the shoulder b, the index will 

 at first move back some 10° or 20°* it will then become sta- 

 tionary for a short interval and afterwards move rapidly forward. 

 The reason of this is that the sudden application of a high heat 

 causes the tube to expand before its eiFect is felt by the in- 

 cluded bar, the consequence is that the bar becomes relatively 

 shorter, and the effect of contraction is produced upon the 

 wheels ; but directly that the influence of the heat reaches the 

 metal, itrapidly overtakes the counter expansion of the tube, and 

 the index immediately moves forward to the point which it would 

 have attained if it had been gradually heated. The reverse of 

 this takes place if it be suddenly taken from a high temperature 

 into the cold air. This is one beautiful testimony of the de- 

 licacy and accuracy of the instrument. It is well known that 

 an analogous effect is produced upon a thermometer under 

 similar circumstances ; if its bulb be placed suddenly in the 

 flame of a candle, or it be otherwise suddenly heated, the 

 mercury will appear to fall in the tube, that is to say, the ex- 

 pansion of the glass momentarily exceeds that of the metal. 



After having ascertained that the effect of the combination was 

 such as I had anticipated, and that the index moved forward 

 regularly in proportion to the heat applied, and returned in 

 cooling to the point from which it set out, my next object was 

 to ascertain, if this effect were perfectly equable, and to obtain 

 the value of the degrees, if possible, in degrees of the ther- 

 mometer. 



For this purpose I procured a cast-iron case, one foot in 

 length, two inches wide, and two inches and a half deep, in 

 which was a partition one inch from the end. In the centre 



