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The thread, /, by which the counterbalance, dd, 
is fufpended, furrounds in like manner the inward 
pulleys B, and H, but in a contrary direction. The 
weight d is about 8 grains heavier than the counter- 
balance, dd, which is fully fufficient to move both 
the indexes, and alfo the regifters; and is fo flight 
a weight, as does not at all impede the rifing of the 
mercury in the tube. 
As the cone d is fupported by the mercury ; when 
it rifes or falls in the tube, the pulleys B and H are 
moved round accordingly ; and as a rife or fall of 
3 inches caufes one revolution of the pulley B, and 
3 revolutions of the pulley H, the indexes A, and E, 
placed on the axes of thefe pulleys, mud be carried 
round the circles b,b,b,b, and g, g, g, g, in like 
manner. 
The index A is moved 4 of the circle b, b, b , b , 
by 1 inch rife, or fall of the mercury, and the index 
E the whole circle g g g g ; which, being about 30 
inches in circumference, and divided into 300 equal 
parts, {hews half a diviflon very perceptibly ; fo that 
the index E marks diftindly the 6coth part of an 
inch rife or fall of the mercury. And by placing 
the regiftersy^/, fig. 1. clofe to the index, it mu ft 
carry one along with it, when it moves ; leaving it 
on its return, to mark the limits of its motion. 
It requires a good deal of nicety and exadnefs in 
making and placing the fridion wheels and pulleys; 
which require to be very light, and the pivots fine 
and fmall. The machinery of this, as alfo of the 
former, was made by Mr. Vulliamy, watchmaker 
to her Majefiy ; which is fo well adjufted, that it 
has 
