16 



than two ; and for the changes of the horizontal force, a bifilar mag- 

 netometer with a thermometer for registering the temperature of the 

 enclosed magnet ; a barometer, either Newman's standard or a por- 

 table, with their attached thermometers ; a dry and wet bulb ther- 

 mometer for ascertaining the humidity of the atmosphere ; and a 

 standard thermometer. The observations were usually made under 

 canvas, and close to the sea-shore. There being usually but one 

 assistant, it was impossible to observe during the whole twenty-four 

 hours ; the hours of observation selected were therefore from 3 a.m. 

 to 9 P.M. inclusive. In Borneo, and during the first four months in 

 Java, there were three assistants, and observations were taken hourly. 



As these observations were made at the different stations in dif- 

 ferent seasons, it was necessary to have the observations of some 

 station for upwards of a year, for the purpose of instituting a com- 

 parison between the changes of one station with another. This was 

 more especially the case with the declination changes, as the times 

 of extreme westerly variation are continually altering. The curves 

 of changes of declination at Singapore were therefore projected 

 for each month of the three years 1843, 1844 and 1845, during 

 which time the observations had been made hourly ; and like- 

 wise for each of the four seasons ; by comparing with these the 

 observations made during the Survey, the author was able to di- 

 stinguish the changes due to geographical position from those due 

 to the season of the year. 



At Singapore, the extreme westerly variation occurs in December 

 at 19 hours, or 7 a.m. ; in January at 20'^ ; in February at 21^ ; in 

 March, or the month in which the sun passes the equator, there is a 

 slight retrogression ; in April the extreme westerly variation is at 

 21^; in July and August at 23^^ ; in September at 22^ ; in October 

 at 21^, and in November at 20 hours. In the winter the extreme 

 westerly variation is at 20 hours ; in spring at 21 hours ; in summer 

 at 23 hoars, and in autumn at 21 hours ; agreeing in this respect 

 with the spring curves, but differing very materially as regards the 

 progression of the needle eastward in the afternoon. The oscillation 

 has much the greatest range during the v/inter months ; autumn 

 comes next, and preserves an almost perfect parallelism with the 

 winter. The spring and summer curves preserve their parallelism 

 during the afternoon. 



The mean curves of each of the three years have an almost perfect 

 resemblance. There are two most decided maxima at 18^ and at 3 

 or 4 hours, and one minimum at 21^. There are besides two other 

 minima at 10^ and I7^ and another maximum at 14 hours, making 

 in all three maxima and three minima. If we turn to the seasons, 

 we find in the winter only one strongly defined maximum and mini- 

 mum. In spring two maxima, one minimum ; in summer and autumn 

 two maxima and two minima. The amount of oscillation is greater 

 in winter than in summer. The sun is a longer time to the south 

 of the line of minimum force than to the north of it. Singapore is 

 in north latitude 1° 18', and the lines of no dip and minimum force 

 cross the meridian of Singapore in latitude 8° north nearly, and 



