Y0L. LXXXVI.] PHILOSOPHICAL TRANSACTIONS. 29 



facility with which molybdaena parts with oxygen is evinced not merely in the humid 

 way, for M. Pelletier found that molybdic acid yielded oxygen to arsenic when dis- 

 tilled with it, and that the latter was converted into a white oxyde. The same is 

 also proved by my experiments on various oxydes distilled with sulphate of ammonia; 

 for the molybdic acid was the only one which could thus be deprived of oxygen, not 

 excepting the tungstic acid, which has been supposed much to resemble that of 

 molybdaena. 



XIII. Observations of the Diurnal Variation of the Magnetic Needle at Fort 

 Marlborough, Sumatra. By John Macdonald, Esq. p. 340. 



These observations were carefully made in a wooden building, having nothing of 

 iron near it. They were taken 3 times mostly every day, at the hours of 7 or 8 in 

 the morning, and at noon, and 4 or 5 afternoon ; from June 27, 1794, till the 

 middle of March 1795- The thermometer was commonly at about 80°. 



It appears from these observations, that the diurnal east variation of the variation, 

 increased from about 7 in the morning till 5 in the afternoon, and that it thence 

 decreased till 7 in the morning. The whole variation was, on a medium, about 

 1° 8' east ; and the change in the variation, between morning and evening, 2 or 3'. 

 It appears in general, that such diurnal variation of the variation as had been observed 

 during thunder, is greater than it ought to have been, caeteris paribus. It has 

 been remarked, that heat weakens the magnetic virtue, and that cold strengthens 

 it. Supposing, with Dr. Halley, the existence of 4 magnetic poles, by blending 

 this supposition with the above principle well ascertained, attempts have been 

 made to account for the diurnal variation of the variation. The south-east mag- 

 netic pole being less heated in the morning, either by the sun or by subterraneous 

 fire, than towards noon, and in the afternoon, and being at the same time, by 

 passing through the meridian of Celebes, nearer Sumatra than the south-west mag- 

 netic pole, it draws to it in the morning the south end of the magnetic needle more 

 powerfully than the other attracts ; and consequently the diurnal variation of the 

 variation ought to be, and actually is, less in the morning than in the afternoon. 

 In the progress of the day, the south-east magnetic pole having become heated, and 

 the south-west pole being at the same time less heated, attracts the south end of 

 the magnetic needle more powerfully than the other does ; and hence the east 

 diurnal variation of the variation appears greater in the afternoon than in the morn- 

 ing. It is found in Europe, that this diurnal variation of the variation is greater in 

 summer than in winter. This seems to point out heat acting on magnets in the 

 earth, as its efficient cause. This was first observed in Europe in 1756, by Mr. 

 Canton ; and the results of the foregoing observations being diametrically opposite 

 to his, with similar effects, afford not a small confirmation of the essential part of 

 Halley's theory. 



From the greatness of the angle of dip of the needle, we are led to suppose that 

 the magnetic poles are fixed within the magnetic nucleus far within the earth's sur- 



