Carpenter v. Longan was a case heard by the United States Supreme Court in 1872. The case involved a dispute between two parties over a contract for the sale of a steamboat. The plaintiff, Carpenter, had contracted with the defendant, Longan, to purchase a steamboat for $2,000. Longan had agreed to deliver the boat to Carpenter, but failed to do so. Carpenter then sued Longan for breach of contract. The Supreme Court held that Longan was liable for breach of contract. The Court found that Longan had failed to fulfill his contractual obligations and that Carpenter was entitled to damages. The Court also held that Carpenter was entitled to recover the full amount of the contract price, plus interest, as damages for the breach. The Court's decision in Carpenter v. Longan established the principle that a party who breaches a contract is liable for damages equal to the full amount of the contract price, plus interest. This principle has been applied in numerous cases since then, and is still used today.

In Carpenter v. Longan, the Supreme Court was asked to decide whether a state court had jurisdiction over an action brought by a citizen of one state against another in which the defendant resided in yet another state. The majority opinion held that it did not have such jurisdiction and dismissed the case. However, Justice Field dissented from this decision and argued that under Article III of the Constitution, Congress has given federal courts exclusive original jurisdiction over cases between citizens of different states regardless of where they reside or are domiciled at any particular time. He further noted that while there may be some instances where concurrent jurisdiction is appropriate for certain types of actions, this should not be extended to all cases involving citizens from different states as it would interfere with Congress’s power to regulate interstate commerce and could lead to confusion among litigants who might find themselves subject to two sets of laws depending on their residence or place of business at any given time.