THE THERMOMETER. 



131 



THE THERMOMETER. 



HEAT, like all other physical agents, can only be measured by its effects, and 

 these effects are very various. The dilatations and contractions which bodies 

 undergo by change of temperature, so long as these bodies suffer no change in 

 their physical state from solid to liquid, or from liquid to gas, or vice versa, form 

 the best and most convenient means of measuring the degrees of temperature. 

 This property has, therefore, been taken as a principle for the construction of 

 instruments for measuring heat, which have been called thermometers and py- 

 rometers ; the former being applied to the measure of more moderate tempera- 

 tures, while the latter have been chiefly applied to determine the more fierce 

 degrees of heat. 



Bodies in every state being affected with a change of dimension by every 

 change of temperature, all are adapted, more or less, to form measures of tem- 

 perature. Solids and gases, being more uniform than liquids in their expan- 

 sions, and having a wider range of temperature without attaining the limits at 

 which they change their physical states, would appear at first view to be the 

 best suited for this purpose. There are other considerations, however, to be 

 attended to, which show that, on the other hand, liquids are best adapted for 

 thermometric indication. The changes of dimension which a solid undergoes 

 by change of temperature are, as has been seen, extremely small, and not easily 

 observed. To appreciate them, it is necessary that their effects should be in- 

 creased by wheels or levers, or other mechanical means ; and such apparatus 

 never fail to introduce error into the result, in proportion to their complexity. 

 Bodies in the aeriform state command, it is true, an unlimited range of temper- 

 ature, without changing their form ; but, on the contrary, their high suscepibil- 

 ity of dilatation and contraction renders them extremely inconvenient in meas- 

 uring any considerable variations of temperature. The changes of dimension 

 of liquids, while they are greater and more easily observed than those of solids, 

 and therefore require no mechanical contrivance for magnifying them, are, on 

 the other hand, less than those of gases, and present a means exempt from the 

 inconveniences of either of the other methods. 



