Comparators for Measures of Length. 2 1 7 



of long ones. The only criticism which I imagine will be 

 urged against this form of construction, is that founded on a 

 doubt whether the contact between the stops always indicates 

 the same measured space. The arm of the pinion has a head 

 of about 2^ inches in diameter. In my own case the sense of 

 touch has been so far cultivated that I am able to make 100 

 successive contacts without a single deviation exceeding 

 .000035 inch, and very few deviations reach .000010 inch. A 

 comparator of this form possesses one decided advantage over 

 all others, viz., that after the stops are once set, any adjustment 

 of the microscope may be made without interfering with the compari- 

 son. The only condition required is that the relation between 

 the stops and the bed shall remain unchanged during the short 

 time required for the comparison. This does not usually take 

 over 10 minutes. 



In order to compare separate subdivisions of the same 

 standard we proceed as follows: 



The stops are set, e. g.^ equal to i decimeter. After the read- 

 ing of the first decimeter has been taken, as indicated above, 

 the bar is then moved along till the first line of the second 

 decimeter forms a contact with the zero of the eye-piece mi- 

 crometer, when at the same time contact is formed with the 

 first stop. Moving the plate to the second stop the reading for 

 the second decimeter is taken. A comparison of the several 

 values obtained with the mean value, will show how much each 

 is in error, provided the entire length is correct. 



For the present, all standards of length made by the writer 

 will be referred to a line yard and meter bar, constructed by 

 the U. S. Coast Survey for Stevens' Institute, Hoboken, and 

 very kindl)^ loaned to me by Professor Mayer. It is marked 

 Y. & M., No. 2. 



Professor J. E. Hilgard, Assistant in charge of the U. S. 

 Coast Survey Office, has kindly communicated to me the state- 

 ment that the yard is standard at 62° F. It was constructed from 

 the yard known as " Bronze 11." The meter was constructed 

 from the iron meter in the possession of the Coast Survey, 

 which was one of four bars from which the platinum bar of 

 the Archives was constructed. It is believed to be the only one 

 of the originals now in existence. There is still some doubt 

 about the exact temperature at which copies of this bar are 

 standard. It is somewhere between 67° and 69° F. A careful 



