The word "Chess"

Wolfram  Alpha (WF) indicates the occurrence of the word "Chess" over time. In case you don't know WF relies on a supercomputer to obtain its answers. As you can see from the graph below, usage seems to be greater in the mid 1600s than in the 1700s. Any idea why this occurs? Recently, the ocurrence seems to be fairly constant. 

Thanks, Walt, interesting subject (and interesting site WF, which I can use for other words as well).

I will try to compare with the number of books and publications around 1640-1660-1680 and around 1740-1760.

The increase  in 1800 can be thanks to a new availability of sources.

It is generally known that world wide interest in chess started in the mid-1600s, than diminished, --  and also that in the 18th century it was far less, to a minimum, due to less good players.

I don't yet understand the big increase in 1800-1850...We will see...

There's nothing before 1640. Why? No texts then?

Yes, Areez, a short answer already : Lucena, Damiano, Ruy Lopez, Salvio and Greco wrote books before approx. 1640.

WF is depending on sources, so if not available, or in other languages, they miss words, can be a reason.

It is also interesting the the occurrence has been growing indicating an increase in the popularity of the game. My guess was that chess was getting less popular because of the rise of video games.