Birt, William Radcliff

(b. Southwark [London], England, 15 July 1804; d. Leytonstone, England, 14 December 1881)

astronomy.

Birt’s capacity for the measurement and analysis of observational data, exhibited in his early studies of the brightness fluctuations of the stars β-Lyra andα-Cassiopeia, which he submitted for publication in the Memoirs of the Royal Astronomical Society, so impressed the society’s president, Sir John Herschel, that from 1839 to 1843 he employed Birt as his assistant in the arrangement and reduction of numerous series of barometric measurements. In the course of this work, Birt discovered large fluctuations in the readings that lent strong support to Herschel’s view that well-defined atmospheric waves were produced by contrary winds blowing across Britain and western Europe.

The results of Birt’s subsequent research on this and other meteorological phenomena, including electrical measurements made at the Kew Observatory, are in the annual reports of the British Association (1844–1849) and in a series of articles in the Philosophical Magazine (1846–1850). By this time Birt had become convinced that the height of the column of mercury in the barometer was a reliable index for forecasting the occurrence of storms and for obtaining one’s position relative to the center of a “revolving storm”(cyclone). The manner in which such data can be used by ships’ captains to provide working rules for steering away from the storm center is described in his Handbook on the Laws of Storms (1853).

Shortly after being elected a fellow of the Royal Astronomical Society on 14 January 1859, Birt began making systematic observations of sunspots, solar rotation, and lunar markings. It is for his work in selenography—he was the Selenographical Society’s first president—which he pursued between 1861 and 1866 at Dr. John Lee’s observatory at Hartwell, Bed fordshire, and thereafter (with the aid of his own seven-and-one-half-inch equatorial reflector) at his small private observatory in Waltham’s Town, Essex, that he is now best remembered. As secretary of the Lunar Committee for Mapping the Surface of the Moon, set up by the British Association in 1865 to revise and supplement Beer and Mädler’s lunar map, he wrote the annual reports up to 1869, introducing in them his own notation for the identification of such small features as craterlets, mountains, and rills.

As an aid to classifying the brightness of the lunar markings, Birt used a scale of lunar tints consisting of twenty-four shades of a single pigment—his homo-chromoscope— which he describes at the end of his monograph Mare Serenitatis (1869). His careful comparison and measurement of photograms and numerous telescopic observations by other leading selenographic experts strengthened his previous conviction that there was a “secular variation of tint” on the floor of the crater Plato, such as might have been caused by eruptive action or chemical activity on the moon. He was always very conscious, however, of the provisional nature of his conclusions, and later expressed the belief that any speculations on such physical changes would require the support of terrestrial analogies from chemistry, geology, and mineralogy.

After 1873 his health began to fail, and in 1877 he stopped observing altogether. Two years later he presented to the Royal Astronomical Society twelve manuscript volumes containing the portion of the lunar catalog that had been completed. He died, after a further rapid decline in his already poor health, at the age of seventy-seven.

BIBLIOGRAPHY

Among Birt’s works are Handbook on the Laws of Storms (Liverpool, 1853) and Mare Serenitatis (London, 1869). Most of his publications are in the Monthly Notices of the Royal Astronomical Society (1859–1872), the British Association Reports (1859–1870), and the Philosophical Magazine (1846–1880). A detailed list, with an indication of each work’s contents, is in Poggendorff, III (1898), 134. In addition, there are ninety-five letters from Birt to Sir John Herschel among the latter’s unpublished correspondence at the Royal Society, London.

An obituary notice is in Monthly Notices of the Royal Astronomical Society, 42 (1882), 142–144.

Eric G. Forbes