«$o Mr. Gilpin’s Observations on the Variation , 
Although a valuable Paper on the diurnal variation of the 
horizontal magnetic needle, by the late Mr. John Canton, 
F. R. S. was published in the first part of the LI st volume of 
the Phil. Trans, for the year 1759, containing a great number 
of observations made at different and irregular times of the 
day throughout the year ; yet, it appeared to me, that if 
the variation were to be observed at short but stated intervals 
of the day for one year, the results would perhaps not only 
prove more satisfactory in determining the times of the needle 
becoming stationary, but would show its progressive and 
regressive motions better, than if observed at irregular inter- 
vals. To effect which, I imposed this laborious task upon 
myself for the space of sixteen months. 
The observations contained in Table I. in sixteen pages, 
viz. from September, 1786, to December, 1787, both inclusive, 
are the results, made at many but stated times of the day, 
and so disposed, that the progress, or regress, of the variation, 
may be readily seen by mere inspection. 
Table II. contains the mean monthly variation for the 
above mentioned times of the day contained in Table I. 
Table III. contains, besides the mean monthly true varia- 
tion, and mean monthly diurnal alteration of variation, for the 
sixteen above mentioned months, the mean monthly true 
variation, and diurnal alteration of variation for many months 
in the year, between the years 1 786 and 1 805 inclusive. 
The numbers put down in Table I. are each of them a 
mean of five observations, and often more. 
Those in Table II. depend on Table I. 
As the observations from which the true variation has been 
given in Table III. between the years 1788 and 1805 were too 
numerous to be all inserted, it has been thought sufficient to 
