41. Akin to this class are some words, originally neuter, that have
the singular and plural alike; such as _deer_, _sheep_, _swine_, etc.

Other words following the same usage are, _pair_, _brace_, _dozen_,
after numerals (if not after numerals, or if preceded by the
prepositions _in_, _by_, etc, they add _-s_): also _trout_, _salmon_;
_head_, _sail_; _cannon_; _heathen_, _folk_, _people_.

The words _horse_ and _foot_, when they mean soldiery, retain the
same form for plural meaning; as,--

The _foot_ are fourscore thousand,
The _horse_ are thousands ten.
--MACAULAY.

Lee marched over the mountain wall,--
Over the mountains winding down,
_Horse_ and _foot_, into Frederick town.
--WHITTIER.



III. Plurals formed by Adding -s or -es.


42. Instead of _-s,_ the ending _-es_ is added--

(1) If a word ends in a letter which cannot add _-s_ and be
pronounced. Such are _box, cross, ditch, glass, lens, quartz_, etc.