To Jason Gridley of Tarzana, discoverer of the Gridley Wave, belonged the credit of establishing radio communication between Pellucidar and the outer world. It was my good fortune to be much in his laboratory while he was carrying on his experiments and to be, also, the recipient of his confidences, so that I was fully aware that while he hoped to establish communication with Pellucidar he was also reaching out toward an even more stupendous accomplishment--he was groping through space for contact with another planet; nor did he attempt to deny that the present goal of his ambition was radio communication with Mars. Gridley had constructed a simple, automatic device for broadcasting signals intermittently and for recording whatever might be received during his absence. For a period of five minutes the Gridley Wave carried a simple code signal consisting of two letters, "J.G.," out into the ether, following which there was a pause of ten minutes. Hour after hour, day after day, week after week, these silent, invisible messengers sped out to the uttermost reaches of infinite space, and after Jason Gridley left Tarzana to embark upon his expedition to Pellucidar, I found myself drawn to his laboratory by the lure of the tantalizing possibilities of his dream, as well as by the promise I had made him that I would look in occasionally to see that the device was functioning properly and to examine the recording instruments for any indication that the signals had been received and answered. My considerable association with Gridley had given me a fair working knowledge of his devices and sufficient knowledge of the Morse Code to enable me to receive with moderate accuracy and speed. Months passed; dust accumulated thickly upon everything except the working parts of Gridley's device, and the white ribbon of ticker tape that was to receive an answering signal retained its virgin purity; then I went away for a short trip into Arizona. I was absent for about ten days and upon my return one of the first things with which I concerned myself was an inspection of Gridley's laboratory and the instruments he had left in my care. As I entered the familiar room and switched on the lights it was with the expectation of meeting with the same blank unresponsiveness to which I was by now quite accustomed.