19 



four hours, viz. at 2 and at 18 hours. The oscillation varies consider- 

 ably; the smallest oscillation being observable at Singapore, where the 

 standard thermometer was placed inside the Observatory, and ex- 

 posed to a current of air passing through the building. The range 

 was greatest at Moulmein and in Sumatra; but at these observa- 

 tories, the instruments being under canvas, the direct influence of 

 the sun's rays was very great. In addition to these thermometrical 

 observations, there was likewise in use a Solar Radiation Thermo- 

 meter, the bulb of which was tinged of a dark purple colour, not 

 absolutely black ; this instrument was placed on a table outside the 

 observing tent, and the bulb beyond the edge of the table exposed 

 to the sun. This thermometer was read off from 19 hours to 4 or 

 5 hours P.M., and the maximum of the day, with the time at which 

 it was observed, recorded. The minimum thermometer (self-regi- 

 stering) was placed at night, and in a similar manner, on the table 

 outside the tent, and the minimum read off in the morning. These 

 observations have been recorded in the Tables, but have not been 

 deemed of sufficient importance to lay them down in curves. This 

 completes that portion of the Survey which relates to the hourly 

 changes of the magnetical and meteorological instruments. 



The author next proceeds to the consideration of the absolute 

 determinations, which formed the principal object of the Survey, 

 viz. Latitude, Longitude, Dip, Horizontal Force and Declination. 

 The method adopted in making these observations was as follows. 

 On arrival at a station, the tent was pitched, and the instruments 

 prepared. These were a six-inch dip circle, a portable decli- 

 nometer, an altitude and azimuth instrument by Robinson, and a 

 chronometer which had been long in use, and was scarcely trust- 

 worthy for a fixed rate. Commencing the following morning, the 

 first instrument set up was the declinometer, and as the suspen- 

 sion-thread was thicker than necessary, to obviate the necessity of 

 frequent renewal, the brass weight for removing the torsion was 

 allowed to swing for a couple of hours. During this interval the 

 observations for dip were completed, and by 9 a.m. the collimator 

 magnet was in the box, the altitude and azimuth instrument in rear 

 of it, and in adjustment with it. Sights were then taken with the 

 sextant and artificial horizon for time, and with the altitude and 

 azimuth instrument in connection with the collimator magnet for 

 declination. From 10 a.m. to 11 a.m. observations of deflection at 

 four different distances, and of vibration, were made with the port- 

 able declinometer, and the telescope of the altitude and azimuth instru- 

 ment, for the absolute value of the horizontal force. At noon circum- 

 meridional altitudes were observed for latitude, and in the afternoon 

 equal altitudes were taken to confirm the observations of the morning. 

 The instruments were then packed up and sent off to the next sta- 

 tion. This was the system adopted whilst travelling ; but at the fixed 

 stations a great number of additional observations were made of hori- 

 zontal force, dip and declination. 



The method of grouping the results at the difterent stations for 

 the purpose of drawing the isoclinal, isodynamic and isogonic lines, 



2* 



