324 Instrument for measuring the expansion of Solid Bodies. 



Art. VIII. — On an instrument proposed for measuring the expan- 

 sion of Solid Bodies, and which may also he used as a Thermom- 

 eter ; by W. W. Mather, A. M., and Lieut. U. S. Army. 



It has long been a desideratum to measure the expansion of bodies, 

 and the changes of temperature, more accurately than these can be 

 done by the instruments which are or have been in use. Every 

 scientific man is aware of the practical utility of a solution of the two 

 points mentioned, and more particularly, in geodesic operations, 

 where the accuracy of extensive surveys is dependent on a rigid de- 

 termination of the length of the base line, and in the true determin- 

 ation and verification of weights, and of measures of length and capa- 

 city. An instrument for such purposes, becomes more valuable in 

 proportion to its accuracy and its capacity, for rigid verification. The 

 mode of determining the expansion of solids employed by Messrs. 

 Lavoiser and Laplace, is of all the methods that have been employ- 

 ed, the least exceptionable, and probably their determinations, as far 

 as they experimented, are close approximations to the true expan- 

 sions.* 



Mr. Hassler's method of measuring the expansion of his measuring 

 rods by means of micrometer screws, is very ingenious, and it is sur- 

 prising how close his approximations are to those of Lavoisier and 

 Laplace, when we consider the possible error arising from the prob- 

 able slight flexture in his long wire rods.f 



Thermometers all labor under an objection, which it has not, hith- 

 erto, been practicable altogether to obviate, and the one that I shall 

 propose, will probably labor in a slight degree, under the same ob- 

 jection, if employed at a higher temperature than the ordinary ranges 

 of atmospheric heat and cold. This objection, in the. thermometers 

 hitherto used, arises from the different rates of expansion of the bo- 

 dies used in their construction, and from there being no means of 

 testing rigidly, the rates of expansion in each individual case. The 

 fact is notorious, that scarcely any two thermometers, however care- 

 fully constructed are strictly coraparablej and hence, the utility of 

 the instrument in minute scientific investigations is much less than 

 might be expected. To the same defect are to be attributed, in 



* A description of their instrument and mode of experimenting ma}'' be seen in 

 Blot, Traite de Physique, or more in detail in the Memoirs de Tlnstitute. 

 t Vide Hassler on the Coast Survey of the U. S. in the Am. Phil. Transactions. 



