194 Ocean Magnetic Observations, 1905-16 



sighting vane LI has been provided with arrangements for two slides, one for the piano- 

 parallel glass with reference line, and the other for the color-glass shade; (h) the vertical 

 ground-glass pieces with reference Unes for the azimuth work are mounted by clamps 

 instead of, as in No. 4, by screws, thus permitting a more permanent adjustment, and 

 eliminating the danger of breakage occasioned by drilling of glass plates; (i) the line of 

 reference on the cover plate is white instead of black, as for No. 4. (For views of No. 5, 

 see PI. 13.) 



Scheme of Horizontal-Intensity Observations. 



In the following scheme of observations for horizontal intensity, the deflection dis- 

 tances, in the order of increasing magnitude, are designated for the upper standard as 

 Ul, U2, U3, and U4; for the lower standard they are similarly, LI, L2, L3, and L4. For 

 distances 1 and 3, the sight line is L2 to 180° and L2 to 0°, and for distances 2 and 4, L2 to 

 270° and L2 to 90°. Observations for horizontal intensity are made in general by using 

 two deflection distances, for example, Ul and LI together with U3 and L3. The scheme 

 for distances 1 and 3, comprising 16 positions, will illustrate the general method of observa- 

 tion. From the readings there results a value of the deflection angle, u, for each distance, 

 outstanding defects of instrumental adjustments being ehminated, as well as possible, by 

 the scheme of observation. For each position there are taken as many readings as circum- 

 stances require, for example, 5, in the sea work. 



Sight line L2 to 180°, north end of deflecting magnet east, distance Ul] 



(( (( u a lono " « « « " east " LI 



„ „ « u Qo'^ , « ^est, " LI Reading vernier A 



u <i « « Qo' " « " « " vpest', " UlJ 



« " " « 0°, " " '< " " west, " US] 



« " 0°, " " " " " west, " L3l„ ,. . , 



« " ""180°; « " " " " east, " L3 p^^'^'"^ ^^™''" "^ 



« « « « igo"^ " " " " " east, " U3J 



Sight Une L2 to 0°, north end of deflecting magnet east, distance U3] 



Readings of the ship's heading on a reference compass (see p. 203) are made by a 

 second observer, .simultaneously with each deflector reading. These compass readings 

 are, of course, necessary only for observations at sea, not on land, theu' purpose being 

 merely to determine the changes in ship's heading during deflector observations. 



The scheme of observation is similar for distances 2 and 4, using the sights stated 

 above. The times of beginning and ending for each distance, as well as the temperatures, 

 are recorded. For low values of H, the longer deflection-distances are used. 



In order to avoid any drag of the magnet card, 2 full minutes are allowed at the begin- 

 ning of observation for each magnet (not distance) after the magnet is in position, as also 

 between each reversal of sights and bowl; a full minute is allowed between all other positions. 



The minimum time required for a half set, from the beginning of reading to the end, is 

 about 8 minutes, but in general, making allowance for interruption and repetition, about 

 10 minutes are required. All possible precautions are taken against setting up motion of 

 the liquid in the bowl by sharp reversals of sights and bowl, as well as to avoid hfting the 

 card off the pivot by the action of the deflecting magnet during reversal of the bowl, the 

 deflecting magnet being removed during such reversals and held sufficiently far away to 

 have no eft'ect. The complete set, using two distances, with repetitions of readings, as may 



