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story:Measuring the Universe

scene:The weather

The weather
The eighteenth century concern with observing, measuring and recording was nowhere more evident than in meteorology. Samuel Horsley commented in 1774: 'The practice of keeping meteorological journals is of late years becoming very general'. The weather was seen to be important for our well-being. 'We shall always search for ways to make observations more exact, both for the sake of agriculture and our health', said Johann Hemmer in 1780. Records were kept for years in the hope of seeing patterns emerge which could have future use.
Images with the text:
Air thermometers and barometers, 1744.
Thermometers and hygrometers, 1744 and chart showing 15 temperature scales.
Both the thermometer and barometer moved from being experimental apparatus to domestic consumer items by the eighteenth century. While thermometers were not essentially different from the ones we use today, their temperature scales had not been agreed. At the start of the century most researchers used their own scale. In 1745 J.T. Desaguliers published a chart comparing the 15 scales then recognised. By the end of the century two were dominant, the Fahrenheit and Reamur. Most scales were based on the freezing and boiling points of water.
Horticulture was a growing interest among the wealthy, inspired in part by new plants being introduced from the colonies. Greenhouses and conservatories needed to be monitored for exotic species. Some thermometers were marked with the ideal temperatures for various plants. In 1782 James Six proposed a 'maximum and minimum' thermometer which allowed the 'greatest and least degrees of heat to be measured in the absence of the observer'. The mercury carried a small index, similar to a pin, which went up as the mercury rose and parked when the mercury level sank again. The thermometer could be reset using a small magnet.
Images with the text:
John Theophilus Desaguliers, scientist and inventor, 1725.
Six's maximum and minimum thermometer, 1780.
Desaguliers' 1745 chart showing 15 temperature scales.
Temperature was not the only quantity that a keen horticulturalist would wish to monitor. Although there were crude hygrometers (instruments to measure humidity) before the 1770s, it was only then that botanist Horace Benedict de Saussure developed his hygrometer, which relied on the expansion of human hair when moist. Human hair is an especially good indicator of moisture and this type of hygrometer was used until the 20th century.
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De Saussure hair hygrometer, 1801-1830.
Horace Benedict de Saussure, Swiss physicist and geologist, c.1780.
Barometers had become fashionable pieces of furniture by the mid eighteenth century. Some attempts were made to improve the poor accuracy of the domestic instrument but these were not always successful. Diagonal barometers had the effect of elongating the scale, but as the mercury surface spread this device was counter-productive. Another use for barometers was in measuring altitude using the fall in pressure with increased height. Towards the end of the century national surveys and the popularity of Alpine mountaineering drove the demand for better portable instruments. Jean-Andre Deluc made a series of improvements in 1770, which developed the barometer into a modern scientific instrument.
Images with the text:
Diagonal barometer, c.1763.
Deluc's portable barometer, 1772.
Barometers, 1772. Engraved plate by G Dheulland from Recherches sur les Modifications de l'Atmosphere by Jean Andre Deluc.
Rain gauges and anemometers
Although they usually recorded temperature, pressure, and rainfall, weather-watchers were also able to measure wind speed. Rain gauges were simple instruments which changed little for over two centuries, as were vertical plate anemometers. These were instruments that measured wind speed by how far a vertical plate was blown towards the horizontal, like a swinging sign. In 1775 James Lind proposed a pressure-tube anemometer, which measured wind speed by the movement of water in a U-tube when the wind was blown down one side. This led to the development of more precise and standardised methods of measurement.
Images with the text:
Rain collector, 1788. Image also shows a lightening conductor and sundial.
Measuring the wind, Greenwich Observatory, London, 1880.
Four-cup anemometer, 1846.
Smeaton's machine for quantifying wind, 1794.

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