This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
If offeror attempts to revoke the offer by sending a later letter or telegram, or by giving notice of revocation in some other way, and such notice reaches the offeree after the letter of acceptance has been mailed and before it is received, such attempted revocation is of no effect.1 Thus an offer was made by telegraph, subject to prompt reply." It was accepted by telegraph twelve minutes after receipt, which telegram was not delivered for an hour. After the telegram of acceptance was sent and before it was delivered, a telegram was sent revoking the offer. The contract was held to be binding.2