THE REVOLUTION IN WATCHMAKING 359 



of these hairsprings, which requires absolute exactness, an 

 indispensable requisite for correct time, was secured by re- 

 peated trials, a spring being found to meet the requirements 

 of the individual balance. Mr. Logan devised a system of 

 tests of springs by a standard balance, and of all balances 

 by a standard spring, and then grading the springs according 

 to strength. Resort to a schedule of gradings indicates at 

 once the proper spring for any balance. 



The minuteness of some of the screws made in a watch 

 factory may be measured by the statement that it takes 

 nearly 150,000 of a certain kind to weigh a pound. Under 

 the microscope they appear in their true character — per- 

 fectly finished bolts. The pivot of the balance wheel is only 

 one two hundredths of an inch in diameter, and the gauge 

 with which pivots are classified measures to the ten thousandth 

 part of an inch. Each jewel hole into which a pivot fits is 

 about one five thousandth of an inch larger than the pivot 

 to permit sufficient play. The finest screw for a small sized 

 watch has a thread of 260 to the inch and weighs one one 

 hundred and thirty thousandth of a pound. Jewel slabs of 

 sapphire, ruby, or garnet are first sawed into slabs one fiftieth 

 of an inch thick, and are shellacked to plates so that they may 

 be surfaced. Then the individual jewels are sawed or broken 

 off, drilled through the center, and a depression made in the 

 convex side for an oil cup. A pallet jewel weighs one one 

 hundred and fifty thousandth of a pound; a roller jewel a 

 little more than one two hundred and fifty six thousandth. 

 The largest round hairspring stud is four hundredths of an 

 inch in diameter and about nine hundredths of an inch in 

 length. 



It is only the finishing department of a watch factory 

 in the United States that requires the service of skilled watch- 

 makers. Even the assembling of a watch is done b}^ others, 

 the hairsprings being selected by girls with the aid of machines 

 and put in on the balance, within an error of ten seconds per 

 hour or four minutes per day, which is readil}^ corrected by 

 the time screws of the balance. The finishing department 

 is of most interest to watchmakers, because it is in this that 

 the movement is adjusted, being put through all the tests 



