On the human heart _a stone_.
--POE.

When God at first made man,
Having _a glass_ of blessings standing by.
--HERBERT.

The roofs were turned into arches of massy stone, joined by _a
cement_ that grew harder by time.--JOHNSON.

[Sidenote: _Like the numeral adjective_ one.]

192. In some cases _an_ or _a_ has the full force of the numeral
adjective _one_. It is shown in the following:--

To every room there was _an_ open and _a_ secret
passage.--JOHNSON.

In a short time these become a small tree, _an_ inverted pyramid
resting on the apex of the other.--THOREAU.

All men are at last of _a_ size.--EMERSON.

At the approach of spring the red squirrels got under my house,
two at _a_ time.--THOREAU.

[Sidenote: _Equivalent to the word_ each _or_ every.]

193. Often, also, the indefinite article has the force of _each_ or
_every_, particularly to express measure or frequency.