apart horizontal lines 673 c and 673 d on one side of the axis 475 if the length of the window 605 to be formed is long the nozzle apertures also 473 and 483 may be located along spaced apart intermediate horizontal lines 673 e 673 h as shown in fig8 referring to fig8 and 9 upon firing of the charges slits or grooves will be formed through the wall of the casing 603 along lines 603 a 603 d surrounding a loose casing plug 603 p or a plurality of plugs 603 p if the nozzle apertures 673 e 673 h are used which then is are removed by removing the apparatus 421 from the borehole and lowering a tool down the borehole with a junk basket or an electro magnet referring to fig1 a tool 731 having an electro magnet 733 coupled to the uphole electrical source 621 is lowered in the borehole 601 to the level of the window 605 the switch 623 is closed to energize the electro magnet 733 to attach and pull the plug 603 p or plugs 603 p from the window 605 and which is are removed from the borehole by removing the tool 731 instead of forming a rectangular window 605 the nozzle apertures 473 and 483 may be arranged for example in a circle to form a circular window 605 as mentioned above the window 605 may be formed through the wall of the casing 603 to provide access to the formation from the borehole 601 for the recovery of petroleum through the window 605 the apparatus of.