These were all comprised within a vast upper hull, which rested upon the lower hull containing the motive power, the only point of contact being an enormous ball-and-socket joint.

Thus, no matter how much the lower hull might roll and pitch and toss, the upper hull remained level and comparatively undisturbed.
Not only were comfort to passengers and security to movable freight gained by this arrangement of the compound vessel, but it was now possible to build the lower hull of much less size than had been the custom in the former days of steamships, when the hull had to be large enough to contain everything.

As the more modern hull held nothing but the machinery, it was small in comparison with the superincumbent upper hull, and thus the force of the engine, once needed to propel a vast mass through the resisting medium of the ocean, was now employed upon a comparatively small hull, the great body of the vessel meeting with no resistance except that of the air.
It was not necessary that the two parts of these compound vessels should always be the same.

The upper hulls belonging to one of the transatlantic lines were generally so constructed that they could be adjusted to any one of their lower or motive-power hulls.

Each hull had a name of its own, and so the combination name of the entire vessel was frequently changed.
It was not three o'clock when the Euterpe-Thalia passed through the Narrows and moved slowly towards her pier on the Long Island side of the city.