CELESTIAL MEASURINGS AND WEIGHINGS. 185 



And the oftener the unit is repeated (when it once be- 

 comes wearisome), the greater is the difficulty of keeping 

 up the necessary attention, and the greater therefore the 

 amount of error to be feared in each case. To diminish 

 this source of accumulating error (besides the saving of 

 time), it is desirable to diminish the number and increase 

 the nicety of these juxtapositions. Hence the utility and 

 convenience of creating an intermediate unit or set of 

 such units or " Base-measuring bars," and of devising 

 some means of juxtaposing or laying them end to end, 

 without the derangement of one by the small shock 

 arising from the contact (however delicately performed) 

 of its successor. These bars should not be so long as to 

 prevent their being conveniently manageable, yet long 

 enough to diminish greatly the requisite number of their 

 repetitions. The bars now actually used for this pur- 

 pose are miracles of ingenious contrivance and delicate 

 workmanship. They are self-compensating for changes of 

 temperature; that is to say, the two fine dots which mark 

 the two extremities of the measure remain exactly at the 

 same distance from each other whatever be the tempera- 

 ture of the bars, which are compound ones of two differ- 

 ently expansible metals combined on a principle devised 

 by the late Lieutenant Drummond. And their repetition 

 is performed, not by driving the end of one against the 

 other, or by laying dot against dot, but by focussing a 

 detached microscope on the more advanced dot, remov- 

 ing the bar and bringing the other dot under the micro- 

 scope to occupy the exact position in the centre of its 

 field (marked by a cross wire) which its predecessor 



