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THE POPULAR SCIENCE MONTHLY. 



meridian of the star to that of the comet. To make this measure, it 

 is customary to fix in the focus of the telescope some uneven number 

 of fine filaments of spider's-web at (say) equal distances apart, and to 

 allow the telescope to remain fixed while the diurnal rotation of the 

 earth carries the body first to be observed iuto the field of the tele- 

 scope and slowly across this. As it crosses each of the threads, the 

 time at which it is exactly on the thread is noted. Now, when the 

 second body enters the field of the telescope (which is supposed to 

 remain fixed in its former position) the times of its passage over the 

 various threads are noted. 



The mean of the times for the first body gives the time at which 

 this body was on the middle thread (these being at equal intervals), 

 while the mean of the times for the second body gives the correspond- 

 ing time for the second body, and the difference of these two times 

 gives evidently the distance which one of them is, east or west, of the 

 other, expressed in time. This may be easily reduced to degrees, etc., 

 by the rule that twenty-four hours is equal to 360 degrees. 



If it were possible for an astronomer to note the exact instant of 

 the transit of a star over a thread, it is plain that one thread would be 

 sufficient; but, as all estimations of this time are, from the very nature 

 of the case, but approximations, several threads are inserted in order 

 that the accidental errors of estimations may be eliminated, as far as 

 possible. The method of making these estimations will be better 

 understood from the two following figures, 1 and 2. Fig. 1 represents 



Fig. 1. Transit-threads in Telescope. 



the reticle of a transit-instrument as it would be viewed by an observer, 

 where twenty-five threads are placed arranged in groups or tallies of 

 five. The star may enter on the left hand in the figure, and may be 

 supposed to cross each of these wires, the time of its transit over each 



