They "would" (theoretically) heat water with the reflected sunlight to generate steam, it would become compressed if within a sealed container (while being heated), then have a nozzle that would release a focused blast of steam onto/into your enemies. Possible, but like it said, no evidence to suggest this. Interesting nonetheless. Like this, but hotter and deadly: It'd be done à la concentrated solar rays: (https://en.wikipedia.org/wiki/Concentrated_solar_power) ...similar to your photo posted (below), but the light shining on a water tank rather than directly at your enemies. Them Romans were a clever bunch, so I wouldn't put it past them... but, again, no conclusive evidence.