A computerised 8-channel data acquisition unit constructed by the Durham University Microprocessor Unit is described and the calibration of this unit is given in chapter 2. A test for the data acquisition unit using a Geiger-Mul1er cosmic ray telescope is described in Chapter 3. In Chapter 4 the main experiment is described. Two plastic scintillation counters each of 0.4 area and 5cm thick in coincidence were used in this experiment to measure the density spectrum of electrons at sea level in the range 20 - 600 electrons per m(^2). The microcomputer data acquisition system is employed to record the scintillator pulse heights. The measured integral density spectrum has been used to calculate the integral size spectrum at sea level. A brief description of the theory of electron-photon cascades and the formation of extensive are showers is give in Chapter 5.A FORTRAN program which uses Monte Carlo method to simulate extensive air showers generated by high energy protons with a given energy is described in Chapter 6. Using the relation between primary energy and average number of electrons arriving at sea level found by Monte Carlo calculation in Chapter 6 an estimate has been made of the integral primary energy spectrum in the range lO(^14)-lQ(^16)eV, and the result is given in Chapter 7.