What happened in the mid 1700's to start a drop? I can understand the drops starting in the 1910's, and then again in the 1960's due to the federal reserve and

What happened in the mid 1700's to start a drop? I can understand the drops starting in the 1910's, and then again in the 1960's due to the federal reserve and the removal from gold standard, but what about the 1700's?