What is the default symmetric cipher?

[Ian Emmons]

What cipher algorithm is used when the --symmetric option is specified but
the --cipher-algo option is not?

I attempted to answer this question myself.  I encrypted a file using each
of the following commands:

    gpg --symmetric --output hello.gpg hello.txt
    gpg --symmetric --cipher-algo 3DES --output hello-3DES.gpg hello.txt
    gpg --symmetric --cipher-algo CAST5 --output hello-CAST5.gpg hello.txt
    gpg --symmetric --cipher-algo BLOWFISH --output hello-BLOWFISH.gpg
hello.txt
    gpg --symmetric --cipher-algo AES --output hello-AES.gpg hello.txt
    gpg --symmetric --cipher-algo AES192 --output hello-AES192.gpg hello.txt
    gpg --symmetric --cipher-algo AES256 --output hello-AES256.gpg hello.txt
    gpg --symmetric --cipher-algo TWOFISH --output hello-TWOFISH.gpg
hello.txt

These commands use all of the cipher algorithms listed when I run the
--version option.  In each case I used the same passphrase.

I expected that the output of the first command would match the output of
one of the others, and that this would tell me what the default algorithm
is.  Unfortunately, the output of the first command matched none of the
others.  What algorithm is the first command using?

Thanks,

Ian