The system class loader does not understand Jars in Jars. It will find a second Jar in the same directory, if the first Jar's manifest lists that Jar.
If you really need to be able to load classes from Jars in Jars, you could write your own class loader to achieve that. Writing class loaders is not a beginner task, but it's not too hard. There may be ready-made code on the Web.
Another approach is to unjar your third-party Jar, then include the resulting files in your Jar. Be sure to comply with any licenses, if considering this. [ January 07, 2008: Message edited by: Peter Chase ]
Betty Rubble? Well, I would go with Betty... but I'd be thinking of Wilma.