294 



THE INCLINED PLANE, WEDGE, AND SCREW. ' 



position to the adjacent division of the limb. The number of complete revo- 

 lutions of the screw necessary to accomplish this will be the number of sec- 

 onds ; and the number of parts of a revolution over the complete number of 

 revolutions will be the hundredth parts of a second necessary to be added to 

 the degrees and minutes primarily observed. 



It is not, however, only to angular instruments that the micrometer screw is 

 applicable ; any spaces whatever may be measured by it. An instance of its 

 mechanical application may be mentioned in a steel-yard, an instrument for 

 ascertaining the amount of weights by a given weight, sliding on a long grad- 

 uated arm of a lever. The distance from the fulcrum, at which this weight 

 counterpoises the weight to be ascertained, serves as a measure to the amount 

 of that weight. When the sliding weight happens to be placed between two 

 divisions of the arm, a micrometer screw is used to ascertain the fraction of 

 the division. 



Hunter's screw, already described, seems to be well adapted to micrometri- 

 cal purposes ; since the motion of the point may be rendered indefinitely slow, 

 without requiring an exquisitely fine thread, such as, in the single screw, would 

 in this case be necessary. 



