HALOID COMPOUNDS OF SILVER, MERCURY, LEAD, AND COPPER. 1129 
sliding bar, one end of which bears upon the first lever, while the other is in contact 
with a short rod of glass E, which moves freely, but water-tight, in a metal stuffing- 
box. The rod of glass thus enters a brass trough F, which can be filled with water or 
melted paraffine, and heated by means of a Bunsen’s burner. The water or paraffine 
is allowed to run off at the end of an experiment by the tap G. IT is a rod of glass 
similar to E, which moves freely, but water-tight, in the metal stuffing-box I. Between 
E and H is placed the rod of substance, the expansion or contraction of which, we 
desire to determine. The end of H external to the trough F rests against the end of 
the micrometer-screw K, which is tipped with agate, and which moves steadily in the 
support L firmly clamped to the iron bar M, which is screwed to the base of the 
instrument N. The head of the micrometer-screw, 0, is graduated into 250 divisions, 
and is figured to 500 ; the graduations are read off against the cross arm P. A plan 
of the levers is shown in fig. 2, where D (the end of which is seen in fig. 1) is the 
movable bar, sliding in a socket It and bearing against the first lever S, which is pivoted 
at T. The lever S bears against a second lever U, pivoted at Y. From the opposite 
extremity of U a very fine steel chain W, such as is employed in watches, passes to a 
vertical axis X, which carries the needle B. The needle is brought back to zero by 
the pressure of the spring Y against the lever U, and also by a fine steel mainspring 
Z, which is fixed to the vertical axis X. a is a small pin to stop the lever U as soon 
as the needle has passed the zero-point by a few degrees. The levers are shown in 
section in fig. 3 ; the lettering is the same throughout. The framework b is of brass, 
as are also the levers; the pivots are of steel. Pieces of glass, c c, are let into the 
levers at the bearing points to diminish the friction. 
The micrometer has threads y^Q-th of an inch apart; hence one complete revolution of 
the micrometer-head is equal to yJo'th of an inch, and a movement of the head through 
one division is equal to -a sooo th of an inch; but it is quite easy to read to half a divi¬ 
sion, and hence to yooTTofh of an inch. The rods inserted between the micrometer and 
the lever are 6 inches long, and from \ inch to ^ inch in diameter; they rested in the 
earlier experiments on light glass rollers placed in the trough F. A rod of the sub¬ 
stance to be examined is inserted between E and H; perfect continuity of the parts is 
established, and the micrometer-head is turned until the needle stands at zero; the 
reading on the micrometer is registered, and the head is then turned until the index 
B has passed to 180° of arc; by again reading the micrometer, the relation between 
the micrometric divisions and the divisions on the scale of C is established. 
By this means it was found that a movement of '0035 inch in the micrometer 
moves the index through 180° of arc. Now since the index is 6 inches long, in 
moving through half a circle its extremity passes over 18'84 inches, and this motion 
is produced by a movement of '0035 inch; hence the levers multiply any motion com¬ 
municated to them 5382 times. The precise value of this index-scale having been 
ascertained, the determination of the expansion of a body by heat becomes an easy 
matter, provided that we are careful to secure perfect rigidity in all the fixed parts 
of the apparatus (by no means an easy task), and are further careful to prevent the 
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