49816.) On the Ventilation of Coal-Mines. 285 
These strata consist-of various kinds of matter, as free-stone, ‘lime- 
stone, indurated clay, coal, &c. and are disposed in beds or layers, 
the under surface of one bearing against, or lying upon, the upper 
surface of the inferior stratum, which last lies or bears against the 
next below ina similar manner. Some of these strata are of con- 
siderable thickness, being often found from 60 to 100 feet, or up- 
wards (nearly uniform) from the upper to the under surface.” ; 
At p. 11, he goes on to say that ‘* a seam or bed of coal is a real 
stratum, which is found to be fully as regular as any of those other 
concomitant strata found in the coal-field, lying above and below the 
coal; or, indeed, of any other of the various strata which compose 
the superficies of our globe. 
“ There are in many coal countries, and in many coal-fields, a 
considerable number of strata or beds of coal, of various qualities 
and thickness, placed stratum super stratum, with a great variety of 
other strata interposed between them; and sometimes different 
strata, or seams of coal, are so near to one another, that two, three, 
or more of them, are cut through, and worked in one pit. 
* Every stratum of coal has some degree of declivity or slope, 
together with a Jongitudinal bearing; and it stretches as far every 
way as other strata which accompany it. 
“« The strata are seldom or never found to lie in a true horizontal 
situation, but generally have an inclination or descent, called the 
dip, to seme particular part of the horizon. If this inclination be 
to the eastward, it is called an east dip, and a west rise; and, ac 
cording to the point of the compass to which the dip inclines, is the 
denomination. The ascent, or rise, is to the contrary point. This 
inclination, or dip, of the strata, is found to hold every where. In 
some places it varies very little from the level; in others, very con- 
siderably ; and, in some, so much as to be nearly vertical. 
** But whatever, degree of inclination the strata have to the 
horizon, if not intercepted by dykes, hitches, or troubles, they are 
always fourd to lie in the regular manner first mentioned. ‘They 
generally continue upon ene uniform dip until they are broken or 
disordered by a dyke, a hitch, or a trouble. If the strata have an 
east dip, they may, by the intervention of a dyke, have on the other 
side an east rise, which is a west dip ; and in general any consider- 
able alteration in the dip is never met with unless occasioned by the 
circumstances last mentioned, 
** Every stratum ina whole range, or class, or coal-field, is spread 
out to a vast extent in an inclining plane, suppose of a mile, or of 
several miles square, like an inclining field, or face of a country. A 
dead level line drawn through that inclining plane is called the 
bearing of the strata; and another line, drawn right across the 
dead level line, or bearing, is called the declivity of the strata, or 
the dip and rise of the strata. In general we can see but a very 
little way from the rise to the dip, or along the line of declivity of 
the strata, because the strata soon dip down out of our sight, and 
generally a great number of other strata come on above them. But, 
