Every sentence of any criminal court importing a capital punishment, if pronounced in Edinburgh or in any other part of Scotland to the southward of the firth or river of Forth, shall specify a day for having the same put to execution, not being less than fifteen days or more than twenty-one days after the date of such sentence; and if pronounced in any place to the northward of the said firth or river of Forth, the day to be so specified shall not be less than twenty days or more than twenty-seven days after the date of such sentence.