In his satire, The Truth, Terry Pratchett talks about what he calls Clacks Towers, basically tall structures with a line of sight from which messages could be relayed at speeds far greater than a physical courier. 

In the stock market, speed of information is often a vital part of obtaining an advantage. And so any means of obtaining sensitive information before the market as a whole gets it is invaluable. 

But Pratchett didn't invent the semaphore-like towers. They came from the 1830s, fully forty years (an age in the white hot pace of development in the Industrial revolution) before the laying of the first "electric telegraph cable" from London to Hong Kong. 

Pratchett's "Clacks" came from the noise the machines made as they moved huge wooden arms around into positions to represent words, the design having been introduced into France and greatly approved of.

And, as with all technology, people with bad intent found a way to profit from it.

Francois and Joseph Blanc were stock market traders in Bordeaux and they realised that they could get information about stock movements in Paris faster than their rivals, if they used the "optical telegraph" as some have dubbed it.

There was not actually a crime. The information they had was public information in Paris and they came by it openly. But it was market manipulation in Bordeaux, some 600km south of Paris,  because they had and could profit from information which was not available to that market. 

If there was any offence at all, it might have been only that they paid the tower operators to give priority to their messages, so ensuring speed of delivery and maximum advantage.