The true test is, whether the object be of a local character, and local use; or, whether it be of general benefit to the states. If it be purely local, congress cannot constitutionally appropriate money for the object. But, if the benefit be general, it matters not, whether in point of locality it be in one state, or several; whether it be of large, or of small extent.

Commentaries on the constitution of the United States (ed. 1851)