2 HEAT OR CALORIC. 



" The steel must be in contact with the mercury, but not at all mer- 

 ged in it." 



" Under these circumstances, if, in consequence of its expansion, 

 by heat, the mercury advance into the tube, the steel moves before 

 it ; but should the mercury retire, during the absence of the observer, 

 the steel docs not retire with it. Hence, the maximum of tempera- 

 ture, in the interim, is discovered by noting the graduation opposite 

 the end of the cylinder nearest the mercury. The minimum of tem- 

 perature is registered by the enamel, which retreats with the alcohol 

 when it contracts ; but, when it expands, does not advance with it. 

 The enamel must retire with the alcohol, since it lies at its margin, 

 and cannot remain unmoved in the absence of any force competent 

 to extricate it from a liquid, towards which it exercises some attrac- 

 tion. But, when an opposite movement takes place, which does not 

 render its extrication from the liquid necessary, to its being stationary, 

 the enamel does not accompany the alcohol. Hence the minimum 

 of temperature, which may have intervened during the absence of 

 the observer, is discovered, by ascertaining the degree opposite the 

 end of the enamel nearest to the end of the column of alcohol." 

 Dr. Hare. 



3. WEDGWOOD'S PYROMETER. 



(a.) This instrument is constructed on a different principle from, 

 that of other pyrometers and thermometers ; still it affords no ex- 

 ception or contradiction to the law of expansion ; it depends on a 

 permanent contraction of certain cylinders of clay in consequence 

 of the application of heat, which operates by expelling water and 

 eventually by causing a chemical union of the alumina and silex of 

 the clay pieces, and an approximation to the condition of porcelain. 



(6.) The cylindrical pieces of clay* are modelled and thrust 

 through a mould, a little flattened on one side baked gently to ex- 

 pel air and moisture, made to fit at between two converging rules 

 of brass, twenty four inches long, distant .at the wider end .5 of an 

 inch, and .3 at the other, and screwed to a brass plate, divided into 

 two hundred and forty equal parts or degrees, each of which is there- 

 fore one tenth of an inch. 



Jc.) Zero of the scale is 1077^ of Fahrenheit , and indicates a full 

 heat, visible in the day light. 



(d.) Each Wedgwood degree corresponds to 130 Fahr. 



(e.) To convert Wedgwood degrees into Fahrenheit degrees; 



Multiply the Fahrenheit degree by 130 and add 1077.5 ; thus the 

 two may be compared. 



* The clay used by Mr. Wedgwood, was from Cornwall. See Phil. Trans. Vols. 

 72, 74, and 76. 



