8 PERCY SLADEN TRUST EXPEDITION 



and a clear sky are essential. The observations should be made in the morning or 

 evening when the sun's altitude is less than 30°, since its motion in azimuth is then 

 less, thus rendering its observation more easy. In the " Sealark " the observations 

 were made at a Kelvin Compass with a card 10 inches in diameter. The ship's speed 

 is of course reduced to as slow a rate as possible to lessen the vibration, and to 

 preserve the geographical position of the ship as nearly as possible. 



(2) Observations Ashore. 



Observations for Horizontal Force and Declination were taken with a Unifilar 

 Magnetometer by Elliot; and for Dip with a Barrow's Dip Circle by Dover. 



Those accustomed to make these observations at instruments permanently erected 

 in a Magnetic Observatory will find that many practical difficulties arise when making 

 a travelling Magnetic Survey of the nature of that carried out in the " Sealark." 

 This is perhaps not the proper place to consider these, but the order of the day's 

 Magnetic Observations may be thus enumerated : 



Theodolite observation for True Bearing, immediately 

 after landing the instruments. 



Erection of tent, and setting up of Magnetometer. 



Observations for Torsion, Declination, and Torsion. 



a.m. observation for Error of chronometer. (Eq. Alts.). 



Observation of Vibrations. 



Observation of Deflections. 



p.m. observation for Error of chronometer. (Eq. Alts.). 



Observations for Torsion, Declination, and Torsion. 



Erection of Dip Circle, and Observation of Dip. 



Removal of tent and magnetic instruments, and re- 

 erection of theodolite on site. p.m. observation for 

 True Bearing. 



In the above list it will be noticed that time is given for two observations, one 

 a.m. and one p.m. for Declination, and only one for Horizontal Force, and for Dip. 

 In southern magnetic latitude (Mauritius), Declination is at its maximum between 



9 and 10 a.m., and again between 2 and 3 p.m. ; and at these times is altering 

 most slowly in its daily curve. 



The mean, therefore, of observations taken near these hours should give the 

 proper mean declination for the position ; disregarding the annual movement. 



Horizontal Force is at its maximum in these latitudes at about Noon, and at 

 its minimum at about 10 p.m. Its curve, however, when plotted, shows that it is 

 at its mean values at about 7 a.m. and 2 p.m.; but it is then altering most rapidly. 



Vertical Force is, like the Declination, greatest at 9 a.m. and least at 2 p.m.; 

 and is at its mean value at about 11.30 a.m. 



The best times for observing each element, if a mean value for the day is 

 desired, derived from observations when it is greatest and least, will be seen, there- 



