Reference numberMS/119/2/107
TitleLetter from Joseph Henry Kay, Magnetic Observatory Rossbank to Humphrey Lloyd, Trinity College, Dublin
DateMay 1842
DescriptionDiscussion on the work that is being conducting at the Rossbank observatory by Kay. Kay informs Lloyd that he has just recently received Lloyd's letter written a year ago, which has provided much needed clarity. Discussion on the declination magnetometer, Kay is certain that he and Captain [James Clark] Ross are in error that the sliders for this bar are to be observed every month for the purpose of swinging the bar, and Kay records reasons for his certainty, and Captain Ross's disagreement. Kay notes that Captain Ross was of the opinion that since the magnet was immoveable, that it was irrelevant what part of the scale was brought to the wire, and that the bar should be vibrated every month.

Kay discusses that he now finds no difficulty in determining the absolute declination. Discussion on the suspension thread of the declination magnetometer, the coefficient of the torsion, and the intensity experiment.

Discussion on the deflections distance. Kay asks Lloyd in regard to obtaining the moment of inertia of the deflecting bar. Discussion on the horizontal force intensity, and the removal of the horizontal force apparatus from the observatory during the experiment of absolute intensity. Kay proposes many uses of an auxiliary apparatus. Kay asks Lloyd's approval regarding his method of vibrating the deflector by counting vibrations right and left.

Kay informs Lloyd that on the first day of every month after June 1841, he has obtained the magnetic axis of the declination bar by putting it into the instrument after it had been stripped and was left untouched for the remainder of the month, therefore Kay argues that all the observations by which the absolute declination is to be deduced is dependent on the determination of the axis. Kay asks Lloyd for his opinion about discontinuing the vibrations of the declination bar, despite Captain Ross’s wish for the opposite.

Discussion on the bifilar or the horizontal force. Kay discusses his confusion with the instructions relating to the changes in the horizontal intensity, however this is now corrected since Lloyd’s letter arrived. Kay readjusted the angle and obtained a better ratio of the force. Kay describes subsequent readjustments and their results on the angle. Using the formula for K, Kay records the coefficient for all their observations until April 1842. Kay asks Lloyd how to obtain W in the equation for the product of two forces, and ration by removing the bar from the stirrup, and using it to deflect the suspended magnet in the declination magnetometer.

Discussion on the vertical force instrument. Kay notes that they were not previously instructed to vibrate the vertical force needle in both planes, and that the plane of motion should be at right angles to the magnetic meridian. Kay informs Lloyd that all the observations at Rossbank observatory and 'HMS Erebus' and 'HMS Terror' observatories have been made from Captain Ross’s opinion that the plane of its motion should be in the magnetic meridian. Captain Ross on his arrival to Rossbank replaced the vertical force instrument with a better instrument, as it had ‘sluggish and capricious movements’. Kay discusses anomalies experienced when using the vertical force instrument. Kay notes that some observations were removed as the vibrations were too great and reading was not possible, however Kay is now able to ‘quiet’ the needle and observations will not be lost in the future. Kay remarks on the good condition of the present needle, and it follows a regular law in its diurnal change. Kay discusses his inability to provide a constant coefficient K for the vertical force instrument.

Kay expresses his enthusiasm to begin temperature observations after receiving Lloyd’s supplementary instructions. He has only made a rough trial using water to change the temperature of the deflector.

Kay notes they ‘have kept 22 term days with observations every two and a half minutes, making 528 hours of uninterrupted observations, and making a mean average of 7392 carefully noted position of each instrument’ at Rossbank observatory.

Kay discusses bad weather conditions in Van Diemen's Land [Tasmania; Lutruwita or Truwana] and its effects. Discussion on the disturbances caused by the aurora. Dependence on the suns position in the ecliptic on the diurnal changes in the declination. Kay shares his uncertainty about the fluctuation of the inclination.

Kay informs the Lloyd that the ‘colonists have been flattered’ at the island being selected for the site of an observatory. Kay mentions his election to the Royal Society.

Communication from Captain [John Theophilus] Boileau at Simla.

Kay’s intentions to conduct a magnetic survey of the island before he leaves.
Physical descriptionInk on paper.

The letter has a hole where the wax seal has been broken.
Access statusOpen
Fellows associated with this archive
NA4898Kay; Joseph Henry (1814 - 1875)1814 - 1875
NA8252Lloyd; Humphrey (1800 - 1881)1800 - 1881
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