Once a letter is placed in a mailbox, it's in the care of the U.S. Postal Service, the gargantuan organization (765,666 employes, 39,327 post offices and stations) that last year confronted rain, heat and gloom of night to deliver140 billion pieces of mail.

How effective are the U.S. mails? The Washington Post conducted an informal survey to answer this question, and the accompanying map illustrates the results.

Letters were deposited in local mailboxes at noon in mid-February in seven cities around the nation and sent to The Post in Washington. Letters were mailed simultaneously from the District of Columbia to the same seven cities. None of the letters was lost or damaged. All arrived within four days, despite the fact that February is second only to the weeks around Christmas as a high-volume mailing period.

In each case, both a typewritten and a handwritten envelope were mailed, to test whether handwritten mail moves more slowly through the heavily automated system.

For a piece of mail to get from here to there, it follows a route worthy of Rube Goldberg. According to George Conrad, public affairs officer for the Washington, D.C., Post Office, mail is collected by a postal collection driver or a dispatch driver and taken to a Management Sectional Center. There, non-metered mail is postmarked (or "canceled") and "faced," a procedure in which envelopes are placed in uniform position, with addresses right side up and stamps in the upper right-hand corner. Metered mail does not require cancellation.

From "postmarking" letters go to "sorting," where they are sorted by the first three numbers of their zip codes. Most mail then is processed by a Letter Sorting Machine (LSM), a $250,000 device boasting 12 consoles from which operators direct the flow of mail. Letters mailed with mechanically printed addresses pass the LSM and go directly to Optical Character Readers (OCRs), which actually read addresses and direct the mail to the correct zip code. Both machines process approximately 30,000 pieces of mail per hour.

The 15 percent to 20 percent of the mail that cannot be handled by OCRs or LSMs is sorted manually.

Sorted mail is dispatched to the Management Sectional Center at the destined zip code. Here the mail is sorted once more, this time according to address. It then is delivered by a local carrier.

Postal Service spokesman Lou Eberhardt said the U.S. system ranks among the world's best, but that comparison is difficult: " Other systems are basically delivering mail in what amounts to countries the size of one of our states. So that while the English and West German systems are very good, you have to be aware of the caveat: England is not as big as Texas."