6 Strong breeze – large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty
7 Moderate gale – inconvenience felt when walking against wind
8 Fresh gale – generally impedes progress
In the early 19th century, naval officers made regular weather observations, but there was no standard scale and so they could be very subjective – one man’s “stiff breeze” might be another’s “soft breeze”. Beaufort succeeded in standardizing the scale.
The initial scale of thirteen classes (zero to twelve) … related qualitative wind conditions to effects on the sails of a frigate … from “just sufficient to give steerage” to “that which no canvas sails could withstand”.
In 1916, to accommodate the growth of steam power, the descriptions were changed to how the sea, not the sails, behaved and extended to land observations. — Wikipedia
The NZ Met Service has a colourful poster of wind measurements describing the terms it uses in our forecasts today. The poster is free to download for personal use.
Some of the terminology has changed since 1947 – “whistling heard in telegraph wires” has become “whistling in wires” and “inconvenience felt when walking against wind” has become “impedes walking” – but otherwise much remains the same.