OF NATURAL PHILOSOPHY. 355 



(388.) Our means of perceiving and measuring mi- 

 nute quantities, in the important relations of weight, 

 space, and time, seem already to have been carried 

 to a point which it is hardly conceivable they should 

 surpass. Balances have been constructed which 

 have rendered sensible the millionth part of the 

 whole quantity weighed; and to turn with the 

 thousandth part of a grain is the performance of ba- 

 lances pretending to no very extraordinary degree of 

 merit. The elegant invention of the sphaerometer, 

 by substituting the sense of touch for that of sight in 

 the measurement of minute objects, permits the 

 determination of their dimensions with a degree of 

 precision which is fully adequate to the nicest pur- 

 poses of scientific enquiry. By its aid an inch may 

 be readily subdivided into ten or even twenty thou- 

 sand parts ; and the lever of contact, an instrument 

 in use among the German opticians, enables us to 

 appretiate quantities of space even yet smaller. 

 For the subdivision of time, too, the perfection of 

 modern mechanism has furnished resources which 

 leave very little to be desired. By the aid of clocks 

 and chronometers, as they are now constructed, a 

 few tenths of a second is all the error that need 

 be apprehended in the subdivision of a day ; and 

 for the further subdivision of smaller portions of 

 time, instruments have been imagined which admit 

 of almost unlimited precision, and permit us to ap 

 preciate intervals to the nicety of the hundredth, or 

 even the thousandth part of a single second.* 

 When the precision attainable by such means is 



* See a description of a contrivance of this kind by Dr. 

 Young, Lectures, vol. i. p. 191. 



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