During the routine investigation of the E and F regions of the ionosphere, it was found that the supposed ground wave was undergoing severe fluctuations. This phenomenon pointed to reflections from regions at low atmospheric heights. In order to separate these reflections from the ground wave, it was necessary to design a device for producing very short pulses and to make a rapid sweep for the oscilloscope. A positively synchronized thyratron modulator instrument gave a regular succession of pulses each lasting for four microseconds. The receiving antenna was in the form of a loop rotatable about both the horizontal and vertical axes. This permitted a weakening of the ground wave, thus producing better resolution of the received pulses. The short pulse receiver consisted of three stages of radio-frequency amplification and a detector operated from a 110-volt alternating-current circuit. A voltage limiting arrangement on the oscilloscope gave a maximum velocity synchronized sweep of 9000 inches per second. With this equipment, several stratified layers from one to 12 kilometers high were discovered, with occasional echoes from 15 to 65 kilometers. Frequencies of 1614, 2398, and 3492.5 kilocycles were used.