These two sentences seem lonely, but in fact they are exciting. They are just low at first and then high. The contrast shows the poet's helplessness and unwillingness. Qianlong - the energy of early spring, when the power is weak; thunder - the festival of the Waking of Insects. This sentence implies that if you don't fly, you will be fine. If you fly into the sky, you will have the momentum, but the time has not come yet, so let's count it as the time to "hide the dragon, don't use it". These two sentences are actually the sentiments often felt when encountering unexpected talents in life, or being in trouble and downfall, but Du Mu wrote them very vividly.