20 REPORT 1860. 



surface were easily detached, and submitted to observation. The reflected and trans- 

 mitted tints were complementary to each other wben held perpendicularly ; they 

 varied when the position was changed and the path of the rays became oblique. By 

 examination under the microscope the films were found to be composed of a series of 

 still thinner films, several of which were required for manifesting colours by trans- 

 mitted light. In some cases as many as sixteen of these thinner lamina were counted. 

 The colour is brightened in proportion to the number of laminae. When two films 

 overlap, the tint produced is the mixture of the two — yellow and blue, for instance, 

 producing green. In general the layers are flat and the colour uniform, but some- 

 times undulated over bubbles, and then the colour is varied. Some specimens with 

 a few bubbles show a difference of colour at the bubble with common light, and with 

 polarized light the black cross and complementary colours appear. 



Electricity. Magnetism. 



On certain Results of Observations in the Observatory of His Highness the 

 Rajah of Travancore. By John Allan Broun, F.R.S. 



The following were noticed by the author. 1st. With regard to the mode in 

 which the diurnal law of magnetic declination varies from place to place, and the 

 probable position and epoch of the line of least diurnal variation near the equinoxes. 

 For this object two stations were chosen — one near the magnetic equator, 90 miles 

 north of Trevandrum, the other about 40 miles south at Cape Comorin, where con- 

 tinuous hourly observations were made during several months about the periods of 

 the equinoxes. The most marked of the conclusions arrived at from these observa- 

 tions were, that the minimum diurnal variation near the March equinox occurred 

 earlier in the year at Trevandrum than at Shertally 90 miles north, and on the mag- 

 netic equator ; that the law presented marked differences at the two places, near the 

 epoch of minimum variation ; and that the difference of the variations at the two 

 stations occurred almost wholly between midnight, sunrise, and noon, the difference 

 between noon, sunset, and midnight being comparatively small. 



2nd. Projected observations were exhibited in proof of the results communicated 

 by the author to the Leeds Meeting of the Association, that the daily mean inten- 

 sity of the earth's magnetism increases as a whole or diminishes as a whole ; so that 

 if at any point on the earth's surface the daily mean intensity increases, it will be 

 found that it increases similarly at all other places in proportion to the absolute in- 

 tensity at each place, allowance being made in cases of great disturbance to the 

 greater value of distuibances in high latitudes. 



3rd. Projected observations were also exhibited, showing that the mean daily 

 easterly declination of the north end of a magnet followed on the whole the same 

 law of variation in both hemispheres, differing from the diurnal variation, where the 

 north end moves east in the southern hemisphere, while it moves west in the north- 

 ern hemisphere. 



4th. The author had investigated the laws of the diurnal variation of the baro- 

 meter within the tropics. He had endeavoured to determine whether the chain of the 

 Indian Ghats had any influence on the great atmospheric semidiurnal wave moving 

 westward. Hourly observations had been made for a month in 1857, at a station 

 on the eastern base of the Ghats, on the highest peak in Travancore, on the western 

 base (all within a few miles), and at Trevandrum 20 miles distant, near the sea shore. 

 Similar observations had been made in 1858, at four stations on the western face of 

 the Agastier Malley, differing by 1500 to 1700 feet from each other in height, in 

 correspondence with the Trevandrum Observatory. In these observations the greatest 

 care was taken to have the best instruments, the times of observations were pre- 

 cisely simultaneous, and instruments of all kinds were observed likely to give results 

 related to the question examined : fifteen observers were employed, and the observa- 

 tions continued hourly during a month. From these observations, it appears that 



