program cooling
      implicit none
c Note that t and T are the same for Fortran
      real r,time,t_max,T,Ts,T_ini,delta_t
      integer pgopen,i,n_steps,i_again

      write (*,*) 'Please type in the cooling rate r and the surrounding
     & temperature Ts :'
      read (*,*) r, Ts
      write (*,*)     'Please type in the time duration  and the initial
     & temperature T_0 :'
      read (*,*) t_max, T_ini

      if(pgopen('/xwin').le.0) stop
      call pgpap (5.0,0.75)

 100  write (*,*) 'Please type in the small time interval delta_t :'
      read (*,*) delta_t

      call pgeras
      call pgenv(0.0,t_max,0.0,100.0,0,0)
      call pglab('Time (min)','Temperature (C)','Cooling Time')

      n_steps = int(t_max/delta_t) 
      T = T_ini
      call pgsci(2)
      do i=1,n_steps
        time = i*delta_t
        T = T - r*(T-Ts)*delta_t
        if (T.le.45.0) call pgsci(3)
        call pgpt (1,time,T,-1)
      end do
      call pgsci(1)

 102  write (*,*) 'Again ? (1=YES;0=NO)'
      read (*,*) i_again
      if (i_again.eq.1) goto 100
      if (i_again.ne.0) goto 102
      call pgclos
      
      end