Page:Proceedings of the Royal Society of London Vol 69.djvu/256

240 Engineer Townsend Set heliostat for corona spectrum, and again for second contact.

Surgeon Nimmo Exposed at object glass of Thompson coronagraph. Observed duration.

Seven men from the " Pigmy " also assisted ; two counting seconds with the aid of a metronome ; one exposing in front of the condensing lenses for the spectroscope ; one exposing in front of the object glasses of the double camera ; and three assisting in handing plate holders to Mr. Dyson and Captain Oldham.

The mode of procedure, which was carefully rehearsed on several occasions previously, was as follows : The observers were stationed at their instruments, and Lieut. Frewen watched the diminishing length of the crescent of the sun on the ground glass of the corona- graph. He had a paper scale on which the lengths of the crescent were marked, computed at intervals of 3 m, 2 m , l m , 30 s , 20", 10 s before totality. At 3 m before totality Mr. Frewen called out " Stand by," and the metronome, which had been carefully rated, was started. At l m before totality he called out " Get ready." At the words " Thirty seconds " Mr. Dyson and Mr. Atkinson drew out the dark slides ready for exposure. At the word " Ten " the exposures for the " flash " spectrum were begun. At the word " Now " given at the moment of totality the time was recorded by Lieut. Oldham and Dr. Nimmo. Six seconds after totality Mr. Dyson and Mr. Atkinson changed the plates of the spectra and exposed for the corona spectrum, Mr. Townsend moving the image on the slit by the heliostat. Mr. Dyson then went to the Thompson coronagraph and made exposures as detailed below, while Lieut. Oldham exposed with the double camera. When the exposures with the Thompson were finished, about 30 s before the end of totality, Mr. Dyson returned to the spectroscope, where he and Mr. Atkinson changed the plates ready for the second flash, and Lieut. Frewen returned to the ground glass of the coronagraph, calling out " Now " at the instant of re-appearance, the times being noted as before by Lieut, and Com. Oldham, and by Surgeon Nimmo.

The Day of the Eclipse. From May 1 to May 1 3 the weather was very fine in the early mornings, but the sky became overcast later in the day. On some of these days the observing conditions at the time of the eclipse were excellent, and on all were moderately good. On May 13, however, there was a heavy thunderstorm, and till May 17 the sky was generally overcast about noon. May 17 was a very wet day, and on May 18, the day of the eclipse, it was completely overcast at 6 h. At 8 h it began to clear, but during totality there was a good deal of light cloud in front of the sun. At 1 o'clock the sky was perfectly clear.

The following readings of the dry-bulb thermometer were taken on the day of the eclipse :