program vols
!Calculates difference in volume of 2 spheres
     implicit none
     real :: rad1,rad2,vol1,vol2
     character :: response
     do
        print *, 'Please enter the two radii'
        read *, rad1,rad2
        call volume(rad1,vol1)
        call volume(rad2,vol2)
        write(*,10) 'The difference in volumes is, ',abs(vol1-vol2)
10      format(a,2f10.3)
        print *, 'Any more? - hit Y for yes, otherwise hit any key'
        read *, response
        if (response /= 'Y' .and. response /= 'y') stop
     end do
     end program vols
!________________________________________________
     subroutine volume(rad,vol)   
     implicit none
     real :: rad,vol,pi
!calculates the volume of a sphere
     pi=4.0*atan(1.0)
     vol=4./3.*pi*rad*rad*rad
!It's a little quicker in processing to do r*r*r than r**3!
     end subroutine volume