NICK CARTER STORIES _Issued Weekly. Entered as Second-class Matter at the New York Post Office, by_ STREET & SMITH, _79-89 Seventh Ave., New York. Copyright, 1915, by_ STREET & SMITH. _O. G. Smith and G. C. Smith, Proprietors._ Terms to NICK CARTER STORIES Mail Subscribers. (_Postage Free._) Single Copies or Back Numbers, 5c. Each. 3 months 65c. 4 months 85c. 6 months $1.25 One year 2.50 2 copies one year 4.00 1 copy two years 4.00 =How to Send Money=--By post-office or express money order, registered letter, bank check or draft, at our risk. At your own risk if sent by currency, coin, or postage stamps in ordinary letter. =Receipts=--Receipt of your remittance is acknowledged by proper change of number on your label. If not correct you have not been properly credited, and should let us know at once. =No. 148.= NEW YORK, July 10, 1915. =Price Five Cents.= THE MARK OF CAIN; Or, NICK CARTER’S AIR-LINE CASE. Edited by CHICKERING CARTER. CHAPTER I. WHAT THE GIRL DID. The girl at the switchboard held her breath. The detective waiting in the business office saw her. The girl at the switchboard was Helen Bailey. The waiting detective was Nick Carter. No man was ever more quick than he to rightly interpret a facial expression. The partition through which he saw her was of glass, or a portion of it, dividing the general manager’s office in the central telephone exchange from the room in which the great switchboards were stationed. There were other girls, half a score of them, seated in front of the innumerably perforated boards. They were too busy to notice one another. Their eyes were intent upon their work. Their deft hands flew from plug to plug, withdrawing some, inserting others. Their frequent, monotonous calls, the noise of the buzzers and the snapping of the rubber-covered plugs were the only sounds to be heard in that busy room. “Hello! hello!” “Number, please.” “The line is busy.” They were like machines, those switchboard girls, human, living, palpitating machines, each a connecting link for others in every phase of life, every calling and vocation, from the gilded mansions of exclusive society to the smoke-begrimed dives of the underworld. They are the servants of all, and, in a measure, the confidantes of all.