GOOD TIME AND ITS ASCERTAINMENT. 521 



astronomical units, while all the time depending upon them for their 

 ascertainment. And the recent changes of using " standard time " are 

 in the same direction. Of course, every place, not just north or south 

 of another, has a different noon. To prevent the confusion resulting 

 from so many " times," our railroads have adopted as noon the mean 

 times of certain standard meridians. These are taken just one hour 

 apart, so that if the new time were universally adopted, the minute 

 and second hands of all correct clocks would be the same over the 

 whole United States, and the hour-hands would differ by one, two, or 

 three hours. In England they have used Greenwich time over the 

 island for many years, and our system is connected with theirs by 

 using for our standard meridians those which are an even number of 

 hours from Greenwich. In Philadelphia, for instance, which is situ- 

 ated on a standard meridian, the time is just five hours later ; so that 

 tidings of an event, happening at noon in London, if telegraphed im- 

 mediately, will reach Philadelphia a few minutes after seven o'clock 

 in the morning. 



The objections to adopting this standard time, in some places, 

 based on the inconveniences of having noon at some other time than 

 when the sun is on the meridian, very much resemble those made in 

 France when the Government substituted mean noon for apparent. 

 In practice we never know when the sun is on the meridian, and if it 

 gets there at 12.30 instead of 12, no one is the worse off, and the 

 methods of living are readily adaptable to it. 



Time being thus dependent on the facts of astronomy, its ascer- 

 tainment is a part of the work of an astronomical observatory. The 

 instrument used for the purpose is a transit-instrument. It consists of 

 a telescope which is mounted, not to be pointed to any part of the sky, 

 but to swing only in the plane of the meridian. It will point horizon- 

 tally, north or south, to the zenith, and to intermediate points. A star 

 in the east or west can not be seen by it. When it crosses the merid- 

 ian, if the telescope is elevated to the proper angle, it will cross the 

 field of view. To determine exactly what part of the field the merid- 

 ian crosses, a spider-thread is stretched in the tube just in front of 

 the eye-piece, which by a very accurate adjustment must be made to 

 coincide exactly with the meridian. Just as the star crosses this 

 thread, or, to speak more accurately, just as the particular meridian 

 of the place passes under the star, the time must be recorded. As 

 there is a possibility of an error in this, several spider-lines are in- 

 serted parallel to this central one, and symmetrically placed on 

 either side. 



The telescope is connected with an axis pointing east and west, 

 working on the tops of two pillars set far enough apart to allow the 

 telescope to swing between them. 



Let us now go through the operation of "taking a transit." The 

 observer, by means of graduated circles, points his telescope to the 



