At present, this blog is being subjected to a very high volume of spam comments, advertising all sorts of products and being a thorough nuisance.
For the next day or so, I am turning off the ability to comment on older articles. I apologise for this, and will enable this again as soon as the spam has subsided.
You should still be able to comment normally on all more recently-published articles.
Update: I have now opened comments for all articles once again, in the hope that the spammers have gone away.