332 HISTOKY OF ASTKONOMY. 



second, which may be measured with scale and dividers. 

 Thus, if we suppose the transit instruments to be all ad- 

 justed to the meridian, and the rate of the clock to be 

 correct, we have obtained the difference of longitude of 

 the stations compared, independently of the tabular place 

 of the star employed, and also independently of the 

 absolute error of the clock. The observers now read 

 their levels, and reverse their transit instruments. The 

 Cambridge astronomer selects a second star, which is 

 telegraphed in the same manner as the first. Thus the 

 error of collimation of all the telescopes is corrected. 

 The other errors of the instruments must be determined 

 by separate observations in the usual manner. Subse- 

 quently a third and a fourth star were selected by the 

 Cambridge astronomer, and telegraphed in the same 

 manner as they passed in succession over the different 

 meridians. These feperiments were made on the 23d 

 of January, 1849, and occupied most of the night. The 

 results were most wonderful, and have opened an en- 

 tirely new field of investigation. Hitherto in transit 

 observations, astronomers had been accustomed to es- 

 timate fractions of a second entirely by the ear, with 

 only such assistance from the eye as could be derived 

 from the rapid motion of the star through the field of the 

 telescope. The error of such an observation, even with 

 practiced observers, frequently amounts to a quarter of 

 a second. But in this new mode of observation, the 

 observer has no use for his ears. The astronomer 

 might in future be made without ears. It is only nee- 



