92 Mr. J. iV. Broun 07i Terrestrial Magnetism. 



I would also refer to a cm-ious fact with reference to the diurnal 

 law of variation of total force, which I have never seen noticed, 

 or indicated as remarkable by any physicist ; on the contrary, it 

 seems to have been so little marked, that Dr. Faraday, in his 

 paper on atmospheric magnetism, has attempted to explain what 

 is exactly the reverse of the fact (Phil. Trans. 1851, p. 115). 

 Whether this is due to the doubt as to the accuracy of the tem- 

 perature correction, or the effects of local influences, I cannot 

 say ; but as the diurnal variation of temperature for the instru- 

 ment which indicates the variations of intensity in this observa- 

 tory is less than three-tenths of a degree Fahrenheit, there can 

 be no doubt here as to the fact that the total intensity is a maxi- 

 mum near 11 a.m., the time when it is a minimum in high lati- 

 tudes. The fact is shown in all the intertropical observations, 

 and was remarked many years ago by the late Mr. Caldecott in 

 this observatory. 



Of course the epoch of maximum intensity varies as we pro- 

 ceed northward and southward. In Scotland it occurs latest in 

 summer and winter (6 p.m.), and earliest in autumn and spi'ing, 

 or when the sun is near the equator. 



I shall at present only note the fact of the coincidence, in the 

 inversion of the European mean laws of disturbance and total 

 intensity at the equator, the maximum force and maximum dis- 

 turbance of force occurring at the same times. 



To enter in detail into the annual laws would occupy too much 

 space, I shall therefore limit myself to a few brief observations. 



Annual law of Magnetic Declination. — I would suggest to mag- 

 neticians in Europe or elsewhere, before they attempt to deter- 

 mine this law, to observe the annual law of movement of an im- 

 magnetic weight (or of a weight with a weak magnet attached) 

 suspended by a silk thread like the declination-magnet. I believe 

 they will thus find the cause of many curious differences between 

 the results at different places, and probably why the annual law 

 seems to depend on temperature. Cassini's observations, which 

 have been so frequently cited, are I think worthless for this de- 

 termination, as it seemed to me was proved in a paper, before 

 referred to, read to the Royal Society of Edinburgh in 1847. I 

 have found here that a simple weight has an annual and diurnal 

 law of movement of its own, whether suspended by a silk thread 

 or metallic wire, and that in a climate where the meteorological 

 conditions are greatly more constant than in Europe. 



Annual law of Force. — You are aware that I have deduced 

 from the INIakerstoun Observations, as early as 1845, that the 

 horizontal force \vas a maximum near the solstices, and a mini- 

 mum near the equinoxes. This law was obtained from each of 

 several successive years' observations, and I confirmed it by an 



