39^ Mr. Gilpin’s Observations on the Variation , 
Table V. contains the dip of the magnetic needle from the 
years 178 6 to 1805. For the first sixteen months, viz. from 
September, 1786, to December, 1787, both inclusive, the dip 
was observed as frequently as the variation, but as there does 
not appear to be any diurnal alteration in the dip, to make it at 
all interesting to communicate so many observations as were 
made, the mean therefore for each month has been thought 
sufficient for insertion. 
To explain the foregoing Table it must be observed, that 
each of the numbers in the four first columns of the above 
Table, are each of them the mean of several means, as ex- 
pressed in the line against those numbers ; and as each of 
those means, are again the mean of five observations at least, 
each of the numbers in the first line, said to be the mean of 
nine means, is therefore a mean of forty-five observations; 
and so of all the rest. 
The numbers in the fifth column, entitled true dip, are the 
means of the numbers contained in the four preceding co- 
lumns in the same line with it. 
The dipping needles used by Norman, the inventor of the 
dipping deedle, who observed the dip at London in the year 
137 6 to be* 71 0 50'; and of Mr. Bond, who observed it in 
1676 to be-f 73 0 47'; not being so much to be depended upon 
as the needles that have been in use for near a century past, 
render the progressive increase of the dip from Norman’s 
time, to the lime of its maximum, somewhat doubtful. But 
Mr. Whiston, whose needle there is reason to believe was 
more to be relied upon, in the year 1720 determined the dip 
* New Attractive, c. 4. 
f Longitude found. 
