The National Weather Service yesterday declared officially what millions of frost-bitten Americans had already surmised: for the eastern two-thirds of the country, the winter of '77 so far has been the coldest "since the founding of the Republic."

It has been the coldest, in fact, since 1758, when an anonymous clerk at the Philosophical Society in Philadelphia began the nation's oldest continuous record of daily temperature readings.

Relying on data gathered by that clerk and scores of other weather buffs - including Thomas Jefferson, Noah Webster and Henry David Thoreau - the Weather Service's National Climatic Center at Asheville, N.C., reported yesterday that average temperatures last month in the East and Midwest were the lowest for any January on record.

The Weather Service has forecast colder than normal temperatures for the next 30 days. If that prediction holds up, the Climatic Center said, this winter will clinch the title.

The average temperature last month along the East Coast was 25.1 degrees, slightly chillier than the previous record, 25.3, registered in 1857.

In the Midwest, last month's average reading was 11.3 degrees, well below the 12.9 degrees of January, 1857.

But records for Washington, D.C, which date back to 1820, show that the Capital has had at least seven Januaries that were colder than the one just past.

The temperature here averaged 25.4 degrees last month, the Weather Service said, a relatively moderate level compared to the 21.4 average recorded in 1856.

Robert Quayle, an official at the Climatic Center who directed the study released yesterday, said data were drawn from government records dating back to about 1800. Earlier records, and reports for areas where the government did not take temperature readings, were drawn from "diaries, garden books, insurance records - whereved people wrote down the temperature," Quayle said.

"The thing that makes this science possible is that through time there have been a lot of people who were weather nuts," Quayle said.

Systematic temperature observations date back to the formidale genius of Galileo Galilei.

In 1592 Galileo reported observations from a "thermoscope" filled with colored water. When that device proved to have obvious disadvantages in winter, Galileo filled it with wine instead to permit measurement of sub-freezing temperatures.

The modern thermometer employing mercury in an enclosed tube and equipped with a standardized scale, was developed by the Dutchman D.G. Fahrenheit around 1710.

The origins of Fahrenheit's unwieldy temperature scale, with the freezing point of water set at 32 degrees and its boiling point at 212, are unclear. Some historians say Fahrenheit initially believed the normal body temperature to be 100 degrees, AND TOOK THAT AS HIS STANDAR body temperature to be 100 degrees, and took that as his standard.

In colonial days, instruments were "pretty accurate, if the guy using them was careful," according to David M. Ludlum, editor of Weatherwise, the journal of the American Meteorological Society.

Jefferson bought a British thermometer in Philadelphia on July 2, 1776, Ludlum said, and two days later began recording daily temperatures, a practice he faithfully maintained until his death in 1826. Jeffersons reading for Independence Day was 76 degrees.

One other President, John Quincy Adams, took daily readings, as did farmers, naturalists, doctors and insurance executives in the first half of the 19th century.

These records, preserved in diaries and business journals, are the source of U.S. weather data up to 1849, when the Smithsonian Institution began the first formal nationwide recording system.

The temperature readings are not mai tained at the Climatic Center in Asheville. "We've got them all in a memory bank so we can computer-massage them when we want to," Quayle said.

But Ludlum, of Weatherwise, who maintains a considerable memory bank in his head, said he thinks "1918 was colder than this year by a whole degree. We had the same thing then, a cold January with warm temperatures about the 7th of February. Then wham - it was colder than ever by Feb. 20 or so."