M. C. Lea on some new Manipulations. 377 
to limit the length of the single degrees, each fifth and tenth be- 
ing drawn beyond them. 
e cleats are then fastened, the three-edged rule is laid with 
one of its edges resting on the tu be, and its ends also are secured 
by cleats (these last are not represented i in the figure). A piece 
4, 
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cer Calibration. 
When a burette contains a portion of liquid, it isa matter of — 
great nicety to determine the division or fraction of a division to 
which the surface of the liquid corresponds. To save the labor 
of holding the paper with the lower half blackened behind the © 
ube, it is convenient to substitute card board, and to cut two 
parallel slits in it so as to form a band, — ‘being slipped over 
the burette, the card maintains its own positi The use 
per blackened on the lower half, gives, po to Mohr, all 
the accuracy desirable. Bunsen, on the contrary, uses a cathe- 
tometer. That the former method is insufficient any one may 
Satisfy himself by placing the card in position, and then moving 
a head slightly in a vertical direction when the black line which 
assumed to mark the surface of the liquid, will be found to 
Sie also, and shosigh the change is but small, it will be found 
that even the sli ightest movement of the eye produces a change 
in the position of the line which gives a difference in results 
easily detected by a good balance. Let us suppose that _ —— 
server notes the position of the black line before an 
Ving a portion of the liquid, he cannot be certain that hise eye akan 
occupied the same relative: position in both cases, unless he takes 
due precautions. Every observer does not possess a cathetom- 
eter, and the following arrangement which is of extreme — 
city, will be found to answer every purpose 
