How to set Margin using rails Prawn so that, header and footer can be set and displayed outside the margins.

Question

HI I dont have an idea of using margin options with Prawn::Document.generate command. And to use margins for the pdf with header and footer outside the margins.

THis is my Prawn pdf code

Prawn::Document.generate("public/test.pdf") do
     page_count.times do |i|
     go_to_page(i+1)
                ask = "public/ashok.jpg"
        image ask, :at => [15, 720], :width => 120
        alert = "public/alert.jpg"
        image alert, :at => [400, 715], :width => 120
        #font("Times-Roman") do
        #text "Travel Log Report", :size => 15, :style => :bold, :align => :center 
        #end
  end
        # footer
      page_count.times do |i|
     go_to_page(i+1)
     lazy_bounding_box([bounds.left+30, bounds.bottom + 20], :width => 100) {
     text "Bypass Report"
   }.draw
      end
end 

end I couldnt set margin within the header and footer, so i could use all other content within the margins for all the proceeding pages.