2i6 POPULAR SCIENCE MONTHLY. 



either end — that is, he can determine fractions of a second with 

 great nicety. 



As a matter of fact, he has such a key at the telescope which 

 he uses to make his observations in taking time, so that when he 

 wishes to record the precise instant in which anything takes place 

 wliich he is viewing through his telescope he has but to press the 

 key in his hand and an extra tooth will be put into the record 

 which the clock is making, somewhere among the regular teeth 

 put in by the beating of the clock. Later, when he takes out the 

 sheet he can see just where the tooth came, and so at what instant 

 it was. If, now, he knows exactly what the instant was according 

 to the true time as given in his almanacs — that is, what his clock 

 ought to have shown at that instant — he can tell how nearly right 

 his clock is. Once knowing how this clock is, it is a simple calcu- 

 lation to find how the clock which sends the signals is running, and 

 to alter it if needed in a manner we shall describe later. 



The observations the astronomer makes use of to determine 

 these instants of time are upon the " clock stars." He uses a 

 rather small telescope, known as a transit. It is placed with the 

 nicest accuracy in a north-and-south line. It can turn over ver- 

 tically, but can not move sideways out of its line. Its alignment 

 is kept perfect by occasionally sighting some small mark a few 

 rods from the observatory, either north or south. 



If the astronomer points this transit, say, halfway up the south- 

 ern heavens and sees a star pass across the center of its field he 

 knows that that instant gives, as it were, the " noon mark " of 

 that star. If it is one of the " clock stars," he knows by his tables 

 what that instant of time is — should be — by his clock. 



We have seen what his means are of comparing his clock and 

 his observations. iJut observe, now, how much pains he takes to 

 get the most exact observations. 



To begin with, he must have calculated to a nicety his loca- 

 tion. The director of an observatory always knows where he is 

 located in a sense that few other men do. The accuracy of a large 

 part of his observations of any kind depends on his first having 

 determined tlie latitude and longitude of his observatory within a 

 very few feet. Then the data given by his tables are all modi- 

 fied, and adapted to conform to his locality. 



There are stretched across in the eyepiece of his transit five 

 spider lines. The central one is on the central line of the field 

 of his instrument. In observing a star for time the astronomer 

 watches it as it is carried by the rotation of the earth past each 

 of these spider lines, and presses his key — that is, makes a record 

 — as it crosses each line. Taking the average of these five ob- 



