360 



PROCEEDINGS OF THE AMERICAN ACADEMY 



To deterraiue the value of one revolution of the screw of the stage 

 micrometer, and more particularly the ratio existing between the vari- 

 ous short standards available, I give the following measures : — 



* Represented to be copies at 15° C. of the standards of the International Bureau of 

 Weights and Measures, at Paris. 



The 5th column contains the limits within which the screw readings were confinedi 

 and the last column is computed by assuming 



the coefficient of expansion of white glass to be 0.00 00 086 for 1° C. 



„ „ „ untempered steel „ 0.00 00 108 „ 



and „ „ „ copper „ 0.00 00 172 „ 



and applying the small differential corrections to the results in the fourth column. 



Whence the successive standards, expressed in terms of the first as 

 unity, aie 



mm. Froment = 1.00000 



mm. Secretan = 1.00119 



U. S. C. S. cm. = 1.00172 

 Brunner Frere cm. = 1.00139 

 Copy „ „ =1.00117 



I wish to thank Mr. R. W. Willson of Harvard College for his 

 skilful aid in making the micrometer readings necessary in the first and 

 second tables. 



