HE A T APPARA TVS. 133 



(a) CHANGE OF DIMENSIONS AND STRESSES OF SOLIDS, AND 

 OF VOLUME OR PRESSURE OF FLUIDS. 



As a rule, bodies expand by heat, but there are many notable 

 exceptions. It is a matter of very great consequence, therefore, 

 to know the laws of expansion ; as they are not only indispen- 

 sable for the scientific purpose of measuring temperatures, but also 

 for many of the most practical operations of ordinary life. 



The co-efficient of Linear Dilatation of a solid at any temperature 

 is the per centage increment of its length when raised to a tem- 

 perature one degree higher/ 1 



In isotropic bodies, such as glass, lead, &c., this co-efficient is 

 practically the same in all directions. In non-isotropic bodies, 

 such as fibrous wood or iron, and crystalline substances of any but 

 the regular system, it has different values in different directions 

 all referable, however, to three principal directions at right angles 

 to one another. In these there may even be expansion in one or 

 two, and contraction in the others, or the other. Hence an inter- 

 mediate direction might be found in which there is no change of 

 length. This was long ago suggested by Brewster for the con- 

 struction of an invariable pendulum. But the exact determination 

 of dilatation of solids is absolutely essential for the comparison of 

 standards of length, for the measurement of the base-lines for an 

 Ordnance Survey, for compensation pendulums, and for many of 

 the most delicate of physical investigations. 



Instruments for determination of the Co-efficient of Linear 

 Dilatation of a Solid in any direction. [Roy, Ramsden, Lavoisier 

 and Laplace, Fizeau, &c.] 



The Co-efficient of Cubical Dilatation of a Solid is the sum of 

 its three principal co-efficients of Linear Dilatation : in Isotropic 

 bodies it is therefore three times that of Linear Dilatation. 



Instruments for measuring the Relative, and thence the Absolute, 

 Dilatation of Liquids. 



As measurements of the Volumes of Liquids require that they 



