Reference numberMS/119/2/80
TitleLetter from Frederick Marow Eardley-Wilmot, Cape of Good Hope Observatory to Humphrey Lloyd, Trinity College, Dublin
Date12 May 1841
DescriptionEardley-Wilmot writes about his uncertainties regarding instruments at the Cape of Good Hope observatory. Eardley-Wilmot informs Lloyd that he is sending the returns for April [not enclosed]. Discussion on the method Eardley-Wilmot has adopted with each instrument at the observatory.

Discussion on the declination. Eardley-Wilmot informs Lloyd that the reading telescope has not altered its position since last month. He will proceed to determine the value of the new scale using the theodolite. The [declination] magnet is removed once a month for absolute measurements, the torsion and the zero points of the scale are found again. Eardley-Wilmot intends to not remove the lens and scale, once he has vibrated the bar on one or two more occasions next month, unless asked to do so.

Eardley-Wilmot discusses his method to obtain temperature corrections at the time of vibrations.

Mistakes comparing the chronometer. Eardley-Wilmot was carrying the chronometer with the magnet, which produced results that show a change of rate. The results of 24 hours work are rendered useless.

Discussion on changes and alterations to instruments. [Walter] Simms did not provide Eardley-Wilmot with a false object glass. However, Eardley-Wilmot borrowed a level from Colonel Lewis, drawing it out to focus can observe without moving the reading telescope. The cylindrical pin used for suspending the torsion stirrup requires great care. Eardley-Wilmot describes reasons for not using the copper quieting apparatus, and instead chooses to use a bar of pure iron.

Discussion on the experiment of deflection, and the value of R. Eardley-Wilmot discusses the value of U, and differed from the value given by [Karl Friedrich] Gauss. Approximate mean time of vibrations of the magnets. Eardley-Wilmot shares his uncertainty regarding temperature, and he is unable to ascribe value to any set of observations on vibration at present.

Discussion regarding the vertical force instrument. Eardley-Wilmot is pleased with its functionality, however, does not know the correction for deviation of axis from the line joining the crosses.

The torsion force of the horizontal force magnet gradually decreased during the last month. Eardley-Wilmot notes that on 10th May 6am the instrument passed 34 divisions in 12 hours. Eardley-Wilmot asks Lloyd’s opinion regarding adjustments to the horizontal force magnet.

Eardley-Wilmot informs Lloyd about his failure to adjust the dipping needle and is waiting for new ones.

After experiments of deflection the magnetic axis of the suspended bar is altered.

Eardley-Wilmot informs Lloyd that he covers his instruments and telescope in dust, and therefore can ascertain whether someone has touched them.

Discussion regarding magnetical instruments. Eardley-Wilmot needs more register thermometers.

Eardley-Wilmot notes that the second lens sent with the declination magnet is not as good, therefore he has been unable to set up a second declination instrument. He is also trying to make a torsion circle.

Eardley-Wilmot discusses his ‘gunner’ as being an ‘able assistant’.

The chronometer is working well.
Physical descriptionInk on paper
Access statusOpen
Fellows associated with this archive
NA6061Eardley-Wilmot; Frederick Marow (1812 - 1877)1812 - 1877
NA8252Lloyd; Humphrey (1800 - 1881)1800 - 1881
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