398 Mr. Gilpin’s Observations on the Variation , 
give a rate of alteration of the variation which could be relied 
on. 
The celebrated Halley thought the variation of so much 
importance, that he made two voyages for the purpose of 
making observations on the variation, to confirm his theory 
advanced in 1683, and soon after he published his variation 
chart. Since his time no better theory than he left has been 
obtained, although it must be confessed that many observations 
have been made at sea by voyagers ; but these observations 
made generally to answer the purpose of the observer at the 
time only, are therefore seldom preserved ; for unless made 
by authority, which rarely happens, they do not often meet 
the public eye ; and it must be from observations made with 
care, and with good instruments, carefully registered, and 
properly arranged, that any real advantage can be derived. 
It is hoped therefore, that in future attention to this subject 
will not be thought beneath those who may have it in their 
power essentially to promote an undertaking so interesting 
to the philosopher, and so valuable and useful to the maritime 
world. 
