ary State, the latter has no
jurisdiction to tax the same.[551] Vessels, however, inasmuch as they
merely touch briefly at numerous ports, never acquire a taxable situs at
any one of them, and are taxable by the domicile of their owners or not
at all;[552] unless, of course, the ships operate wholly on the waters
within one State, in which event they are taxable there and not at the
domicile of the owners.[553] Only recently airplanes have been treated
in a similar manner for tax purposes. Noting that the entire fleet of
airplanes of an interstate carrier were "never continuously without the
[domiciliary] State during the whole tax year," that such airplanes also
had their "home port" in the domiciliary State, and that the company
maintained its principal office therein, the Court sustained a personal
property tax applied by the domiciliary State to all the airplanes owned
by the taxpayer. No other State was deemed able to accord the same
protection and benefits as the taxing State in which the taxpayer had
both its domicile and its business situs; and the doctrines of Union
Refrigerator Transit Co. _v._ Kentucky,[554] as to the taxability of
permanently located tangibles, and that of apportionment, for
instrumentalities engaged in interstate commerce[555] were held to be
inapplicable.[556]

Conversely, a nondomiciliary State, although it may not tax property
belonging to a foreign corporation which has never come within its
borders, may levy on movables which are regularly and habitually used
and employed therein. Thus, while the fact that cars are loaded and
reloaded at a refinery in a State outside the owner's domicile does not
fix the situs of the entire fleet in such State, the latter may
nevertheless tax the number of cars which on the average are found to be
present within its borders.[557] Moreover, in assessing that part of a
railroad within its limits, a State need not treat it as an independent
line, disconnected from the part without, and place upon the property
within the State only a value which could be given to it if operated
separately from the balance of the road. The State may ascertain the
value of the whole line as a single property and then determine the
value of the part within on a mileage basis, unless there be special
circumstances which distinguish between conditions in the several
States.[558] But no property of an interstate carrier can be taken into
account unless it can be seen in some pla