ULLOA'S voyage to. south AMERICA* |^^ 



Thefe obfervatlons, thus compared, fhew the errors to which navigators are liable, 

 for want of attention in making choice of proper needles, which they fliould be careful 

 to procure, not only well made and exaQ:, but alfo ftriftly tried with regard to their in- 

 clination to the true meridian, before they venture to depend upon them in any voyage. 

 In this point, Spain is guilty of a notorious negled, notwithftanding it is evidently the 

 fource of a thoufand dangerous errors ; for a pilot, in correcting the courfe he has 

 fteered, in making ufe of a compafs whofe variation is different from the true, will con- 

 fequentlv find a diiference between the latitude by account and the latitude obferved ; 

 and to rnake the neceffary equation according to the rules commonly received in failing 

 on points near the meridian, he mull either increafe or diminifh the diftance, till it 

 agrees with the latitude, whereas in this cafe the principal error proceeded from the 

 rhomb. The fame thing happens in parts where it is apprehended there may be cur- 

 rents ; which often occur in failing when the latitude by account, and that by obferva- 

 tion, difagree ; though in reality the water has no motion, the difference proceeding 

 entirely from making ufe of another variation in the courfe, than that of the needle by 

 which the fhip is fteered ; as was the cafe with me in failing from Martinico to Curafao, 

 and likewife of all the artifls on board the fhip. Another error incident to navigators, 

 though not fo much their own, is to fleer the fhip by one needle, and obferve the va- 

 riation by another ; for though they have been compared, and their differences carefully 

 obferved, their motions being unequal, though at the beginning of the voyage the dif- 

 ference was only a certain number of degrees, the continual fridion of the forpier on 

 the pivot, renders the point of the needle, on which it is fufpended, more dull than 

 the other, which is only hung when they make obfervations, being at all other times 

 kept with the greatefl care ; and hence proceeds the change obfervable in their differ- 

 ences. In order to remedy this evil, all needles intended to be ufed at fea fhould be 

 equally proper for obferving the variation ; and the obfervation made with thofe before 

 placed in the bittacle ; and, to improve the charts of variation, Ihould be touched iij 

 the fame manner, and adjufted to the meridian of place, where the exact variation is 

 known. Thus obfervations made in the fame places by different Ihips, would not be 

 found fo confiderably to vary ; unlefs the interval of time between two obfervations be 

 fuch as to render fenfible that difference in the variation, which has been obferved for 

 many years paft, and is allowed of by all nations. 



Thefe are the caufes of the manifeft difference between needles ; there may be others, 

 but this is not the proper place for enumerating them. 



CHAP. II. — Defcription of Carthagena, 



ON the 9th of July 1735, we landed, and Don George Juan and myfelf imme- 

 diately waited on the governor of the place. We were informed that the French acade- 

 micians were not yet arrived, nor was there any advice of them. Upon this informa- 

 tion, and being by our inflrudtions obliged to wait for them, we agreed to make the 

 befl ufe of our time ; but were unhappily deflitute of inftruments, thofe ordered by 

 His Majelly from Paris and London not being finilhed when we left Cadiz, but were 

 forwarded to us at Quito foon after our arrival. We, however, fortunately heard that 

 there were fome in the city, formerly belonging to Brigadier Don Juan de Herrera, 

 engineer of Carthagena ; by thefe we were enabled to make obfervations on the latitude, 

 longitude, and variation of the needle. We alfo drew plans of the place and the bay 

 from thofe of this engineer, with the neceffary additions and alterations. 



2 In 



