99 
A letter was read from Samuel Crompton, Esq., M.D., 
&c., accompanying a fine series of photographs of distin- 
guished astronomers, and of the principal astronomical 
instruments of the Paris Observatory, which he had received 
from x4.dmiral Mouchez, the Director of the Observatory, 
and which he now sent for inspection in the belief that they 
would be interesting to the members of the Society. 
“ Note on an attempt to analyse the recorded Diurnal . 
Ranges of Magnetic Declination,” by Balfour^ Stewart, 
M.A., LL.D., F.R.S., Professor of Natural Philosophy at the 
Owens College, and William Dodgson, Esq. 
1. It is well known that Professor Rudolph Wolf has 
endeavoured to render observations of sun spots made at 
different times, and by different observers, comparable with 
each other, and has thus formed a list exhibiting approxi- 
mately the relative sun-spot activity for each year. This 
list extends back into the seventeenth century, and is 
unquestionably of much value. Nevertheless, it must be 
borne in mind that we possess no sun-spot data sufficiently 
accurate for a discussion, in a complete manner, of questions 
relating to solar periodicity before the time when Schwabe 
had finally matured his system of solar observations, which 
was not until the year 1832. 
We have, however, a much longer series of the diurnal 
ranges of magnetic declination. Now, these are already 
well known to follow very closely all the variations of sun- 
spot frequency, being greatest when there are most, and 
least when there are fewest spots, and it may even be 
imagined that such ranges give us a better estimate of true 
solar activity than that which can be derived from the 
direct measurement of spotted areas. 
The long-period inequalities of the diurnal range of 
magnetic declination are thus, we may imagine, precisely 
