What is the first weapon the Martians use to kill the humans (it’s a box-like apparatus)?

From the text, we might infer the Martians are using robots.

At that time firing was going on across the common, and it was said the first party of Martians were crawling slowly towards their second cylinder under cover of a metal shield.

Later this shield staggered up on tripod legs and became the first of the fighting-machines I had seen. The gun he drove had been unlimbered near Horsell, in order to command the sand-pits, and its arrival it was that had precipitated the action. As the limber gunners went to the rear, his horse trod in a rabbit hole and came down, throwing him into a depression of the ground. At the same moment the gun exploded behind him, the ammunition blew up, there was fire all about him, and he found himself lying under a heap of charred dead men and dead horses.