I see gem in "gem list" but have "no such file to load"

Question

I am on Ubuntu10

sudo apt-get install ruby1.9.1-full

then download sources of rubygem 1.3.7 and install it

sudo ruby setup.rb

then, for example, install sinatra

sudo gem install sinatra

Finally open irb and type

require "rubygems"
require "sinatra"

and get error

LoadError: no such file to load -- sinatra
    from (irb):2:in `require'
    from (irb):2
    from /usr/bin/irb:12:in `<main>'