98 REPORT—1858. 
Let there be constructed two similar sets of right cylinders, say each set, 
six to twelve in number, all of equal height (a) and of the same sort of 
material, but varying in diameter in each set, with a uniform decrement 
from the greatest to the least. 
Convenient dimensions for earthquake observations of mean intensity, will 
be such, that the cylinder of largest diameter shall have its altitude equal to 
three diameters, or b=o, and that the cylinder of least diameter shall have 
« 
its diameter one-third of that of the greatest one, or bat Any number of 
cylinders of intermediate diameters may be interpolated between ; and the 
greater the number, the more accurate the instrument becomes. A series of 
six to ten in each set will, however, be sufficient for any purpose. For 
observation of shocks of extreme violence, larger diameters, in proportion to 
altitude, should be chosen for all the cylinders. 
The material of the cylinders is not important, cast iron, stone, pottery, or 
other substances at hand, whose arrises will not crumble away by being 
overthrown, may be used; but no material will be found more convenient 
than some hard heavy wood, of uniform substance, straight grain, and equa- 
ble specific gravity, from which the cylinders can be formed in the lathe, 
and their bases brought perfectly square to the axis with facility. 
Upon any horizontal and solid floor let two planks be placed, as in fig. 6, 
with their directions in length respectively lying N, and 8. and E. and W., 
Fig. 6. 
each plank to be about 3 inches in thickness, and in width equal to the dia 
meter of the largest cylinder, and its length such that the set of cylinders, — 
when placed upright and equidistant thereon, shall have a space greater than — 
the altitude between each. Thus, if the cylinder of largest diameter have 
b=0'5 of a foot, the length of plank will, for a set of six, as in the figure, 
be about 12 feet. These base-planks being fixed, level, and solid, the floor is 
