In Leviathan, why is the Leviathan called "artificial?"

The answer to this can be found in the very first
paragraph of the introduction to this work.  There, Hobbes argues that the leviathan is
artificial because it is made by people to imitate something in
nature.

Hobbes argues that anything that people make to
imitate nature is, by definition, artificial.  He says that the leviathan is artificial
because it is made by people to try to imitate a human being.  He then goes on to list
various parts of the leviathan and to note which parts of the human being they
correspond to.  As a couple of examples, he says that the sovereignty of the leviathan
is its soul and the laws are the will of the being.  There are also more physical parts,
such as the government officials, who are the joints of the
being.

Because the leviathan is made by human beings in
this way, it is an artifical human being on a very large scale.