Fire of Isengard ?

[The Might]

Good post Manwe
Indeed the fact that this "bomb" had been placed in a narrow place would have increased its effect much more, but still, it could be that both these ideas are correct. Saruman might have enhanced it with his magic, and had perhaps planned that it should be set in a narrow place. Perhaps the Orcs entering the Deep that way were just used to make sure the defenders would block the culvert.
Now, I wouldn't exactly call the device magic...it was just a proof of more advanced technological invention in a not so developed world. I am sure that the indians thought the weapons of the conquistadors were using some kind of magic as well, but I believe we must differenciate between magic and superior technology. We see Saruman was definitely interested in enhancing and inventing things, we see proof for this in Isengard and in the Shire.