LINEAR MEASUREMENT. 217 



There is an insurmountable difficulty in converting line-measure to 

 end-measure, and therefore it is most desirable for all standards of 

 linear-measure to be end-measure. 



Line-measure depends on sight aided by magnifying glasses ; but 

 the accuracy of end-measure is due to the sense of touch, and the 

 delicacy of that sense is indicated by means of a mechanical 

 multiplier. 



In the case of the workshop measuring machine, the divisions on 

 the micrometer wheel represent ten-thousandths of an inch. The 

 screw has twenty threads to an inch, and the wheel is divided into 

 500, which multiplied by twenty gives for each division the ten- 

 thousandth of an inch. 



We find in practice that the movement of the fourth part of a divi- 

 sion, being the forty-thousandth of an inch, is distinctly felt and gauged. 

 In the case of the millionth machine, we introduce a feeling-piece 

 between one end of the bar to be measured and one end of the 

 machine, and the movement of the micrometer wheel through one 

 division, which is the millionth of an inch, is sufficient to cause the 

 feeling-piece to be suspended or to fall by its gravity. 



The screw in the machine has twenty threads, which number multiplied 

 by 200 the number of teeth in the screw wheel gives for one turn of 

 the micrometer wheel the four-thousandth of an inch, which multiplied by 

 250 the number of divisions on the micrometer wheel gives for each 

 division one-millionth of an inch. The sides of this feeling-piece are 

 true planes parallel to each other, and the ends both of the bars and 

 the machine are true planes parallel to each other, and at right angles 

 to the axis of the bar ; thus four true planes act in concert. In 

 practice we find that the temperature of the body exercises an 

 important influence when dealing with such minute differences, and, 

 practically, it is impossible to handle the pieces of metal without 

 raising the temperature beyond 62. I am of opinion that the 

 proper temperature should be approaching that of the human body, 

 and I propose that S$ Q fahr. should be adopted, and that the 

 standards and measuring appliances should be made and kept in a 

 room at a uniform temperature of 85 fahr. 



In many workshops we hear the workmen speak in such vague 

 terms as a bare sixteenth or full thirty-second, but minute and 



