574 MR J. A. BROUN ON THE 
The first epoch is probably one year too late; the second may not be far from 
the truth (19.); the third is 1:3 years too late; the fourth, 2°2 years too soon ; 
the fifth, 3-5 years too late; the sixth is 0°3 years too late; and the remaining 
epochs are probably not far from the truth (Arts. 30-32). The most marked 
differences are those for the otherwise irregular period 1795-1801. 
23. Does the movement shown by Gilpin’s observations from 1795 to 1801 belong 
to a true period ?—While the very different and sometimes absolutely opposite 
results, obtained by many celebrated observers, from the needle, supported ona 
steel pivot, in the period 1770-1780, immediately before the commencement of 
GILPIN’s observations, have thrown doubt on all observations made with the same 
kind of instrument since, I have already noted some proofs of the accuracy of the 
conclusions deduced from GILPIN’s series, and the agreement in the first instance 
at least with Cassini’s observations, showing nearly the same epoch of maximum 
diurnal oscillation. The doubt, however, is not wholly removed by these other 
proofs, and it is increased by the fact, that since systematic observations have 
been made with more perfect instruments, we have seen no appearance of so 
short a period, and especially have seen no period in which the whole variation 
of the diurnal oscillation was less than 1’, as in the case in question. 
On the other hand, we must also observe that we have no case in which two 
minima of nearly equal value, belonging to the same epoch, are separated by an 
interval of 5°5 years like those for 1795-0 and 1800°5. It acquires only a glance 
at the curves projected, Plate XX XIX., to see the weight of this fact. We 
have, however, another method of determining whether a period probably existed 
of such a character as that shown by GILPIN’s observations. 
24. The general agreement between the epochs of maximum . and minimum 
sun-spot frequency and diurnal range of the magnetic needle has been already 
referred to (6.): we shall leave at present the evidences of this agreement 
during the last half century, and consider what is known of the former pheno- 
menon in the periods immediately preceding. Our knowledge of sun-spot 
observations during the last as well as in a great part of this century is due to 
Dr Wotr. Unfortunately, it is just at the time when we have the most need for 
complete series of sun-spot observations that they are most wanting. From 
1790 to 1815 was a period when men seem to have had their attention turned 
too strongly towards the earth’s surface for them to be able to examine with care 
that of the sun. Dr Wo yr's “ relative” spot numbers for this period can there- 
fore be considered only as rude approximations in some cases, and perhaps as 
doubtful guesses in others. They cannot, however, I think, be considered without 
any value on account of these defects; they contain all the information we 
possess on spot frequency for the time.* 
* Dr Lamont has criticised some of the epochs which Dr Wozr considers certain (“sicher”), and has 

