292 Major 8. G. Burrard on 
Radium rays may be inserted provisionally between Light- 
shock and Lamplight in the above order. 
Conclusions. 
1. The density of the image produced on a plate by 
exposure to radium rays (8 and ¥) increases to a critical 
value and then decreases, at first rapidly and afterwards very 
slowly, until a time is reached when the image is totally 
reversed. 
2. Spark-images are at first obliterated by radium rays 
which do not cause such a great density as that of the 
spark-images obliterated. 
3. With prolonged exposure radium rays reverse spark- 
images. 
I am greatly indebted to Mr. Hayles, of the Cavendish 
Laboratory, for carrying out many of the experiments 
described in this paper. 

XXXI. On Deflexions of the Plumb-line in India. By Major 
S. G. Burrarp, R.E., Superintendent of the Trigonometrical 
Surveys of India*. 
le the Philosophical Magazine for January 1904, the 
Rev. O. Fisher gives his reasons for thinking that the 
Indian pendulum results do not support my suggestion, that 
a chain of excessive density, many hundreds of miles long, 
lies hidden in the Earth’s crust in Upper India. As Mr. Fisher’s 
paper has been read by many with interest, I would ask to be 
allowed to explain my reasons for having omitted to refer to 
the pendulum results in the original paper on the subject. 
Those reasons may be stated as follows :— 
1. The correctness of the Indian pendulum results has 
been questioned by the highest living authorities (wide 
Colonel Clarke’s ‘ Geodesy,’ and Professor Helmert’s report to 
the International Geodetic Conference, 1901). 
2. The supposed hidden chain of excessive density is 
between 1000 and 2000 miles long; one line of pendulum 
stations crosses this chain at right angles. We cannot get any 
fair idea of the mass of a chain from the results of a single 
traversing line. 
3. The pendulum stations on this line of observation are 
too far apart to allow of conclusions being formed. 
Mr. Fisher’s conclusion that there is a marked negative 
variation of gravity over the whole of India, cannot be 

* Communicated by the Author. 
