54 



HEAT OR CALORIC. 



The process being repeated, the walls were brought into the perpen- 

 dicular position, and if necessary, could have been curved inward. 



L. u. K. 



THERMOMETERS, CONSTRUCTION, USE, &IC. 



1. The common thermometer, and most pyrometers, operate upon 

 the principle of expansion. 



(a.) The thermometer ivas probably invented by Sanctorio, an 

 Italian physician of the seventeenth century. His thermometer was 

 merely a ball blown on the end of a glass tube, and inverted in a 

 fluid ; it was consequently subject to the pressure of the atmosphere, 

 a change in which might cause a movement of the fluid, although 

 the temperature should be stationary. This thermometer is entirely 

 unfit for being used in fluids still it is very useful, as an air ther- 

 mometer, for measuring minute variations of temperature. 



Air Thermometer of Sanctorio, on a large scale. 



The bulb of a mattrass is sup- 

 ported, by a ring and an upright 

 wire, with its neck downwards, 

 so as to have its orifice beneath 

 the surface of the water in a small 

 glass jar. A heated iron being 1 

 held over the mattrass, the con- 

 tained air is so much increased in 

 bulk, that the vessel being inade- 

 quate to hold it, a partial escape 

 from the orifice through the water 

 ensues. On the removal of the 

 hot iron, as the residual air regains 

 its previous temperature, the por- 

 tion expelled by the expansion is 

 replaced -by the water. 



If in this case the quantity of 

 air expelled be so regulated, that 

 when the remaining portion re- 

 turns to its previous temperature, 

 the liquid rises about half way up 

 the stem, or neck, the apparatus 

 will constitute an air-thermometer. For whenever the temperature 

 of the external air changes, the air in the bulb of the mattrass must, 

 by acquiring the same temperature, sustain a corresponding increase 

 or diminution of bulk, and consequently, in a proportionable degree, 

 influence the height of the liquid in the neck. This thermometer is 

 very sensible and would be very accurate, but that it is influenced 



