TARIATION OF THE COMPASS. 389 



diate direction, by Mr. Craskell, the island engineer, 1760, were 

 and Mr. Simpson, both eminent surveyors. But, in "^"^e without 

 • r ,, ,. 1 . ,. , . ^1 notice of the 



these maps, the magnetical meridian only is represented, variation. • 



Neither the magnetical variation nor the true meridian is 

 mentioned : the island's place on the globe, as to latitude 

 and longitude, is not given. In short, the true meridian 

 has never been noticed, nor the quantity of variation as- 

 certained, nor the variation even mentioned, nor f the 

 latitude and longitude, observed by any surveijor* or 

 engineer in Jamaica, but myself. 



Although the discovery of the variation's not varying, 

 in Jamaica, is established on the clearest evidence v^^ith- 

 out the aid of other data, yet it is highly gratifying to' 

 tind Dr. Halley, as it were, confirming it to the minutest 

 accuracy, as will appear from the recital of the following 

 observations of Mr. Long, in his History of Jamaica. 



'' The variations of the magnetical needle were ob- Quotatloa 

 *' served by Dr. Halley to be very small, near the equa- shewing the 7 

 *' tor. I have seen no account of them for this island, j^co ^^ j^^^g J 

 '' that can be relied upon; but, if observations should been 6 deg.anti; 

 " be faithfully made here, they would probably confirm^ ^ cast— 

 '' his opinion. According to Mountain's chart, con- 

 ^' structed in the year 1700, from Dr. Halley's tables, 

 '^ the variation at Port Royal then was about 6 1 degrees 

 •^ east. But, as in most parts of the world it is found 

 " to be continually either increasing or decreasing, so 

 '^ we may reasonably conclude, that it may have altered 

 •^' in both respects very much during this long interval 

 •' that has passed since the construction of the chart." 



The magnetical variation, ascertained by me, and laid__^^j j^ .^ , 

 down in my maps of Jamaica, is 6| degrees east. same now 



I leave to others, better qualified than 1 am, to inquire (1806). 

 and to point out, what improvements natural philosophy 

 may derive from this discovery ; which I hope may bean 

 acquisition to scienc?. 



I am afraid I have been, too prolix. But the import- 

 ance of the subject, and my desire to remove even the 

 shadow of any doubt that might be suggested, will, I 

 trust, be admitted as my apology. 



I have the honour to be, &c. 



JAMES ROBERTSON. 



Vol. XV. — Supplement. Gog 



