PRC68.comprc68.com/I/pdf/1922_08_Heliograph_5in_MarkV_med.pdfThese expedients are only necessary in...
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PRC68.comprc68.com/I/pdf/1922_08_Heliograph_5in_MarkV_med.pdfThese expedients are only necessary in exceptional circumstances when proper stability cannot be obtained without them