A LARGE sunspot of rapid development crossed the central meridian on April 16–1. When photographed at the Royal Observatory, Greenwich, on April 13, at 10 h. TJ.T., the spot had an area of 150 millionths of the sun's visible hemisphere; twenty-four hours later the spot, which had become of complex structure, had increased to 1,000 millionths. This is the largest spot seen during the past four months. Greenwich also reports a considerable magnetic disturbance recorded at Abinger as having begun suddenly on April 17 at 1h 55m. There would appear to be a definite association between the big sunspot and the terrestrial magnetic storm.