Royal Society. 343 



time the instruments were observed at some of the stations; and for the 

 purpose of ascertaining whether this really was the case, the curves 

 of the barometer, corrected to 32° of temperature, were projected 

 at some of those stations where observations had been carried on for 

 more than a month, and likewise, in addition, corrected for tension 

 of vapour, thus giving the oscillation of the column of dry air. 

 There is certainly not the same regularity perceptible as in the oscil- 

 lation of the barometer, yet there is considerable uniformity in the 

 oscillation of the pressure of the column of dry air, the maximum 

 generally occurring at 19 or 20 hours, and the minimum at 2, 3 or^ 

 hours. The range is likewise considerably greater. 



The tension of vapour was generally a mininmm at 18 hours, 

 when it amounted to 0*8 of an inch. It then increased to 2 hours, 

 when it might amount to 0'9 of an inch, and then gradually dimi- 

 nished, having but one maximum and one minimum in the 24; 

 hours. In the islands of the Archipelago the air is nearly saturated 

 with moisture, and at Singapore the maximum difference between 

 the dry and wet bulb at 1 or 2 p.m. amounted to not more than 7°, 

 and then gradually and uniformly diminished till sunrise, when the 

 difference was rarely more than a degree. There appears to be 

 one maximum and one minimum of the dry and wet bulb thermo- 

 meter, the former at noon or 1 p.m., the latter at 18 or 19 hours; 

 the greatest and least differences between the two thermometers oc- 

 curring, as has been observed, likewise at these hours. 



In addition to the above, a standard thermometer by Newman 

 was observed ; one maximunj and one minimum occur in the twenty- 

 four hours, viz. at 2 and at 1 8 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 tlie 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, l)ut have not been 

 deemed of sutticient 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 tiie consideration of the absolute 

 determinations, which formed tiie princii)al object of the Survey, 

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

 The metliod adopted in making these observations was as follows. 



