irOW STANDARD TIME IS OBTAINED. 217 



servations, lie makes the possible error very small. But, in addi- 

 tion to this, lie also usually makes observations on at least four 

 clock stars, wliicli gives liim twenty observations to average up 

 and determine by. As lie inspects the record of these observa- 

 tions which has gone upon the chronograph sheet along with the 

 clock beats he is able to determine, after proper calculations, how 

 his clock stands. 



Such observations are made every three or four evenings, and 

 thus the clocks are not given time to get far out of the Avay. It 

 is not usual for a good clock to show a variation of more than half 

 a second. If the astronomer finds that his clock which is send- 

 ing the time is running a fraction of a second slow, he goes to it 

 and lays on the top of the pendulum bob a minute clipping of 

 metal, which is equivalent to shortening the pendulum an infini- 

 tesimal amount. When he takes his next observation he discov- 

 ers how his clock has been affected, and again treats it accord- 

 ingly. Thus the time that is sent out automatically by the clock 

 is kept always correct within a small fraction of a second. Those 

 who receive the time sometimes arrange electro-magnets near the 

 pendulums of their clocks, which act with the beats of the observa- 

 tory clock, and their attraction is enough to hold or accelerate 

 the pendulums as needed to make them synchronize with the ob- 

 servatory clock. 



It will be seen that the means of obtaining exact time involve 

 a very considerable outlay, and that the services of highly trained 

 men are needed. The public is thus greatly indebted to the rail- 

 roads, telephone companies, and other corporations which usually 

 bear the expense of securing standard time. It is probable, how- 

 ever, that from motives of scientific pride no observatory would 

 undertake to charge for this anything like what would be exacted 

 for such rare service in any department of the commercial world. 



It is worth while to note that even with such perfect clocks 

 and favorable conditions it is still impossible to secure perfect time- 

 keeping. Add to this the fact that it is not usual for those who 

 send out the time, after it has been received from the observatory, 

 to pay much heed to variations, even of several seconds, in their 

 master clocks, and we see why it is a disheartening task to keep 

 the best watch as near the second as the owner would fain have it. 

 In the first place, the watch could hardly be made to keep such 

 time if kept still in an unchanging temperature; secondly, it is 

 still less capable of it when subjected to the jolting and changes 

 of temperature it encounters when carried; and, thirdly, the 

 means of obtaining time with sufiicient exactitude are rarely avail- 

 able to the general public. 



TOL. LTI. 18 



