
DECENNIAL PERIOD OF MAGNETIG VARIATIONS, ETC. By Al! 
17. Gilpin’s Observations.—Although this series (1786-1805) began after 
Cassinr3, I have considered the latter first for reasons which will soon be obvious: 
The whole difference between the conclusions of Drs Lamont and WoLr 
depends on the interpretation of the magnetical and sun-spot observations made 
between 1787 and 1818. GuiLPIn employed a needle with an agate cap (carefully 
turned by the well-known maker Narrn) resting on a steel point. The instru- 
ment was made under the supervision of CAVENDISH,* and every precaution 
was taken by GILPIN to obtain the true position of the needle at rest by attract- 
ing it frequently to both sides. He observed from 6 a.m. to 10 P.M. in all the 
months of the years 1877 and 1798, but only during from two to. seven months in 
the other years. 
In order to deduce the best possible results the means of the ranges for the 
months on which observations were made in each year were compared with the 
mean of the ranges for the same months in the years 1787 and 1793 ; the differ- 
ences applied to the mean of the ranges for all the months in these two years 
gave the approximate mean range for each year;+ the results are as follows :— 




Year. Bs ss Ranges. Year. eet Ranges. 
1787°2 12 14:84 1796-5 5 7-21 
1787-5 12 14:97 1797°5 5 7°53 
1788°5 4 13°83 1798°6 5 7:29 
1789°5 2 11-88 1799°5 5 717 
1790°5 2 11°93 1800°5 5 6°75 
1791°5 3 11-41 1801°5 5 769 
1792°5 7 9°13 1802°5 5 8:23 
1793°5 12 8°43 1803°5 5 9°19 
1794°5 4 701 1804°5 5 8:13 
1795°5 5 695 1805°5 5 8:17 


These quantities are projected Plate XXXIX., and from the curve we 
obtain the following epochs and ranges— 
Maximum 1787°5=14'97 Minimum 1795'1= 690 
5 1797 7= 7°60 ra 1800 5=6 :75 
55 18035= 9:20? 
18. Dr Wotr considers that GILPIN’s observations show a minimum in 1796, 
whereas Dr Lamont cannot conceive how a minimum in 1796 can be derived 
from these numbers; he adds, “In fact, they show no period whatever, which 
is easily understood when we remember that a needle on a steel point was 
*- See the description by Cavenpisa, Phil. Trans. 1776, p. 385. 
+ Phil. Trans. 1806, p. 416. The ranges'for each month were determined from a mean of observations 
made at those times of the day when the declination was considered greatest and least. Generally 600 
observations were made in each month. 
VOL. XXVII. PART IV. 7M 
