1. It is the natural frequency of emission of the hydrogen atoms in space and was discovered by us in 1951;
    这个频率是空间氢原子释放的自然频率，是在1951年被人类发现的。

2. In the late 1940s, engineer Percy L. Spencer of the Raytheon Company was experimenting with high-frequency (高频率) radio waves.
    20世纪40年代后期，雷神公司的工程师珀西·L·斯潘塞正在从事高频无线电波实验。

3. Fatal accidents have decreased in frequency over recent years.
    近年来死亡事故发生的频率已经下降.