                                                                   
                                                 15                                  12              
               12                                                                         
                        12                                                
                                          12                                                                                                                               
                                                                                                 1830 1840                                                                                                                   
                                                                                                                                                                                                                                       
                                                                                                                                                                                                         
                                                                                                                                                                                                           
                                                                       
              
                