IT HAS NOT ESCAPED notice here that the Celsification of Washington is picking up speed. You may remember that last summer some of the time-and-temperature signs in the city began offering readings in Celsius as well as Fahrenheit. This revolution in thought and custom was initiated by the Eastern Liberty Federal Savings and Loan Association, and since then the movement has spread gradually. But now it has taken a major lurch forward. The Chesapeake and Potomac Telephone Company is not a corporation to be left behind in anybody's technological upheaval, and its weather reports now offer you the bad news in Celsius as well as Old Familiar. We can take it for granted that, following the phone company's swing, we are about to be engulfed once again in progress.

Last summer we made a frivolous reference to the new degrees as Centigrade. That brought us a stiff letter from a reader who claimed that Centigrade is an obsolete term used only by the undereducated, and that all informed people now speak only of Celsius. It is not immediately clear to us why Centigrade should now be a dead word, since after all Celsius himself died more than two centuries ago. But God forbid that anyone should think The Washington Post out of date. We shall drop Centigrade from our vocabulary altogether - at least until another letter arrives telling us that the trend has reversed among the avant garde. It is comforting to know that Centigrade and Celsius both mean the same thing.

But does it advance the happiness of humankind to be told, on a bleak winter morning, that the temperature is - 3 degree instead of 27 degree Old Style? A uniform worldwide system of weights and measures makes a great deal of sense, as you know if you ever tried to screw a metric bolt into a nut with the traditional, or Olde English, thread. But the case for Celsius seems to reside solely in the principle of consistency.

If we're going to be consistent (a new and dangerous idea), then we say let's go the whole hog. The metric-and-Celsius crowd often forget it, but the revolutionary age and spirit that brought us grams and meters also brought us a rational and orderly calendar. It begins on Sept. 22, the day after the involuntary departure of Louis XVI from the French throne, for that is the anniversary of the founding of the First Republic. This calendar proceeds at a stately pace through 12 equal months of 30 days each. At year's end there are five civic festivals (six on leap year) devoted to things like Virtue, Genius, Opinion and so forth - worthy objects, all of them, which this newspaper has long supported as a matter of irrevocable policy.

The months had charming and memorable names devised by the poet Fabre d'Eglantine. Fabre was otherwise something of a skunk and came to a bad end - on the guillotine, in fact. (This observation is offered as a matter of historical interest only; it is not intended as a reflection upon the phone company, or other proponents of the Celsius system.) The past month, on the calendar in question, would have been Frimaire, from the word for frost. Very accurate, this year. The current month would be Nivose, suggesting snow. Another bull's eye.

Each of these months was divided into three weeks of 10 days each. You could use the same calendar year after year, without having to change them as we wastefully do today. It also cuts down the number of weekends, which is good for economic productivity. You get only three Sundays a month, instead of the four or five granted us by the irrational, irregular and disorderly old Gregorian calendar. The shortage of Sundays became a sore point, as it turned out, and the calendar hardly survived the First Republic itself. The moral seems to be that a little rationality sometimes goes quite a long way.