Page:Popular Astronomy - Airy - 1881.djvu/79

Rh of the times may be effected. One of them is by the use of an instantaneous signal of light. If I fire gunpowder on a high mountain, and if my assistants observe, from the two places where the instruments are placed, the time when the flash of gunpowder is seen, I can compare the clock at the two places. One person observes the clock-time at the one place when the flash occurs, and another observes the clock-time at the other place when the flash occurs; and therefore, as soon as a letter can be sent by post from one place to another, I know how much one clock is faster or slower than the other. Another method is, by conveying watches, or small chronometers from one place to another. For instance, an expedition was arranged by myself some years since, to observe the difference of time between Greenwich and Valentia, on the south-west coast of Ireland. I had thirty chronometers carried backwards and forwards more than twenty times from Greenwich to Valentia, to compare the clocks. The chronometers were conveyed by railway carriages, by steam boats, by mail coaches, and by Irish cars, between Greenwich and the west of Ireland. By means of these I was enabled to compare the clocks at the two places; and by transit observations made with the assistance of these clocks, and with proper calculations, the times of the transits of the stars were compared, and therefore, I got the inclination of the planes of the meridian. By a survey-triangulation, extending in the east and west direction from Greenwich to Valentia, the distance was known in yards; and, knowing the distance, and knowing the inclination of the planes, the whole circumference of the parallel

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