that very same morning , when the sky was reddening before sunrise , we had slipped swiftly through still-sleeping vienna , leaving it a couple of hours later a mere patch of smoke against the blue hills of the wienerwald on the horizon ; we had breakfasted below fischeramend under a grove of birch trees roaring in the wind ; and had then swept on the tearing current past orth , hainburg , petronell ( the old roman carnuntum of marcus aurelius ) , and so under the frowning heights of thelsen on a spur of the carpathians , where the march steals in quietly from the left and the frontier is crossed between austria and hungary .