midnight
It had been arranged , among Captain Wiltsey and his helpers , that they were to give a certain signal when they discovered the dynamiters at work , and then the boys would set off their flashlights and begin to work their hand cameras . The automatic one , of course , would need no attention , provided the miscreants went near enough the net-work of strings to break one and so set the mechanism in motion . But that was problematical , and , as Joe said , they would have to `` trust to luck . '' And so the preparations for receiving the midnight callers went on . Joe and Blake worked in silence , making ready for their part in it . All about the boys , though they could neither see nor hear them , were Uncle Sam 's men -- soldiers , some of them -- stationed near where , so rumor said , the attempt was to be made to explode the dynamite . `` We really ought to have another helper , '' said Blake , thoughtfully .