Fire of Isengard ?

[Mänwe]

Originally posted by Boromir88

“Now onto Saruman's 'Fire of Orthanc' where I believe he also used magic to enhance the explosives he used at Helm's Deep.”

An interesting point, though I was wondering if I might be allowed to take another angle on this, and say, no, he did not use magic. In a more practical sense the culvert in which they placed the crude device was small, made more so by that fact that the defenders of Helm’s Deep had blocked it so that the orcs were unable to come through.

“They gathered such small boulders and broken stones as they could find to hand, and under Gimli's direction the Westfold-men blocked up the inner end of the culvert, until only a narrow outlet remained. Then the Deeping-stream, swollen by the rain, churned and fretted in its choked path, and spread slowly in cold pools from cliff to cliff.” – (Chapter 7, Helm’s Deep)

This coupled with the fact that the culvert is by design already a confined space, would have served to contain the explosion, so that the force was not dissipated to the sides wasting the explosive energy, but being contained would have been amplified, thus generating such a destructive force as was seen; blowing the subsequent breach in the Deeping Wall.

Also concerning your quotation of the letter, I have been slightly confused by the topic of magia and goeteia. And perhaps I have misunderstood what you mean, you say that Saruman uses magic to enhance the explosion, then go on to quote magia;

“The basic motive for magia - quite apart from any philosophic consideration of how it would work - is immediacy: speed, reduction of labour, and reduction also to a minimum (or vanishing point) of the gap in time between the idea or desire and the result or effect.” – (Letter #155)

But isn’t the magia here the actual explosive device itself, and is therefore not magic, nor is there any magic enhancing it; this enhancement being magia, for it is the gunpowder as you say, which has been to;

Originally posted by Boromir88

“…break down the defenses of Helm's Deep as quick as was possible.”

[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.