void draw_ell(Double_t dx=20.)
{
   gSystem->Load("libGeom");
   TGeoManager *geom = new TGeoManager("myGlobe", "Globe");
   TGeoMaterial *mat = new TGeoMaterial("vacuum",0,0,0);
   TGeoMedium *med = new TGeoMedium("vacuum", 1, mat);
   // Create the top volume of your geometry
   TGeoVolume *top = geom->MakeBox("TOP",med,200,100,100);
   geom->SetTopVolume(top);
   // Now make just some spheres
   TGeoSphere *sphere1 = new TGeoSphere(0.,20.);
   TGeoSphere *sphere2 = new TGeoSphere(0.,40.);
   // Define some scale transformations on the desired axes
   TGeoScale *scale1 = new TGeoScale(1.,1.,2.5);
   TGeoScale *scale2 = new TGeoScale(1.,1.,0.2);
   // Apply it to your speres to make ellipsoids
   TGeoScaledShape *scsph1 = new TGeoScaledShape(sphere1, scale1);
   TGeoScaledShape *scsph2 = new TGeoScaledShape(sphere2, scale2);
   // Make some real objects out of them and give some colors
   TGeoVolume *sph1 = new TGeoVolume("GLOBE1",scsph1,med);
   TGeoVolume *sph2 = new TGeoVolume("GLOBE2",scsph2,med);
   sph1->SetLineColor(kRed);
   sph2->SetLineColor(kBlue);
   // Position the 2 ellipsoids so that they intersect
   top->AddNode(sph1,1);
   top->AddNode(sph2,1,new TGeoTranslation(dx,0.,0.));
   // Close the geometry
   geom->CloseGeometry();
   // Set the precision of the mesh
//   geom->SetNsegments(40);
   // Draw the top volume; note that just the sphere is visible
   top->Draw();
   gPad->GetView()->ShowAxis();
   top->Raytrace();
}