a single- lens Micrometer, 121 



to be t¥o of an inch in diameter, the magnified image of which 

 occupied fifty divisions of the scale, when it was at the distance 



of 16 fi inches, and hence one division = ^77-^ = 1^* 



Since any error in the original estimate of this wire must per- 

 vade all subsequent measures derived from it, the substance 

 employed was pure gold drawn till fifty-two inches in length 

 weighed exactly five grains. If we assume the specific gravity 

 of gold to be 19,36, a cylindrical inch will weigh 3837 grains, 

 and we may thence infer the diameter of such a wire to be t^o 

 of an inch, more nearly than can be ascertained by any other 

 method. For the sake of rendering the scale more accurate, 

 a similar method was in fact pursued with several gold wires, 

 of different sizes, weighed with equal care ; and the subdivi- 

 sions of the extei'ior scale were made to correspond with the 

 average of their indications. 



In making use of this micrometer for taking the measure 

 of any object, it would be sufficient at any one accidental posi- 

 tion of tlie tube to note the number on the outside as deno- 

 minator, and to observe the number of divisions and decimal 

 parts which the subject of examination occupies, on the interior 

 scale, as numerator of a fraction expressing its dimensions in 

 proportional parts of an inch ; but it is preferable to obtain 

 an integer as numerator, by sliding the tube inward or out- 

 ward, till the image of the wire is seen to correspond with 

 some exact number of divisions, not only for the sake of 

 greater simplicity in the arithmetical computation, but because 

 we can by the eye judge more correctly of actual coincidence, 

 than of the comparative magnitudes of adjacent intervals. 

 The smallest quantity, which the graduations of this instru- 



MDCCCXIII. R 



