1894.] Terrestrial Refraction in the Western Himalayas. 217 



a table is given of the declination and dip at Ascension Island for 

 the epoch 1834 1890, in which the change of the dip from north 

 to south occurs between the years 1842 1839. The same change is 

 not only shown on the magnetarium but also the amount of the dip 

 and declination for the epoch 1834 1890. 



The correlation of the maximum rate of change of the inclination 

 with the minimum rate of change of the declination about the nodes 

 of the magnetic equator is well seen in the observations, and is also 

 demonstrated in my paper (prop. XVIII) and on the magnetarium. 



III. " Terrestrial Refraction in the Western Himalayan Moun- 

 tains." By General J. T. WALKER, C.B., R.E., F.R.S. 

 Received January 13, 1894. 



In the operations of the Great Trigonometrical Survey of India it 

 is customary to determine the coefficient of refraction by reciprocal 

 vertical observations between contiguous stations on the sides of all 

 the principal triangles, and also as many as possible of the secondary 

 triangles. 



[The sum of the reciprocal vertical angles, plus the angle at the 

 earth's centre, would be exactly equal to 180 if there were no refrac- 

 tion ; the excess gives the sum of the refractions in both the angles, 

 and half of it is taken as the amount for each angle. March 2.] 



The values of the coefficient thus obtained for the operations in 

 the Western Himalayas between the meridians of 73 and 80 east 

 of Greenwich have been grouped together for comparison in succes- 

 sive ranges of 2000 ft. of altitude between the elevations of 5,000 and 

 21,000 ft. above the sea level. The operations happen naturally to 

 have been divided into two sections ; for the regions lying between 

 the great snowy ranges on the southern face of the Himalayas and 

 the plains of India were first completed, and some time subsequently 

 the still higher regions to the north, extending up to the Karakoram 

 and Kuenlun Ranges, which look down on the plains of Turkestan. 

 The first portion appertains to what is called the N".W. Himalayan 

 Series, the second to what is called the Kashmir Triangulation. Thus 

 the values of the coefficients of refraction were obtained separately for 

 each section, and the results are shown in the following table, where 

 the heights have reference to the middle points of the sides of the 

 triangles, of which the number is given for each group : 



