# File lib/ai4r/neural_network/backpropagation.rb, line 254
      def update_weights
        (@weights.length-1).downto(0) do |n|
          @weights[n].each_index do |i|  
            @weights[n][i].each_index do |j|  
              change = @deltas[n][j]*@activation_nodes[n][i]
              @weights[n][i][j] += ( learning_rate * change + 
                  momentum * @last_changes[n][i][j])
              @last_changes[n][i][j] = change
            end
          end
        end
      end