I have a brother-in-law who teaches economics at a British university. I have discussed this kind of problem with him, and he says, the only logical way to make comparisons between different eras is to choose some object or commodity which has kept the same intrinsic value over the years. In other words, something which satisfies people today in the same way it did then.

His favorite standard is a pint of beer!

So the question becomes, how many pints could you buy with £40 in 1770, and what would it cost to buy the same number of pints today? That will be your answer.