PRESIDENTIAL ADDRESS. 425 



deserves to be equally held in respect with that of his great uiiLle Prince 

 Henry, was the calling together of the Committee, or ' Junta,' of learned men 

 to consider the best means of finding the latitude when the Pole-star was 

 too low to be of service, to decide upon the most approved form of instru- 

 ment for the taking of observations, and to furnish suitable tables of 

 declination, &c., for the computations. Equipped ■with the new tables, which 

 may, perhaps, be considered the first Nautical Almanac, and the simplified 

 astrolabe, the Portuguese navigators started on the famous voyages, with a 

 much better chance of properly fixing positions than their predecessors. The 

 vernier had not yet been invented, and so the difficulty of obtaining accurate 

 readings of the circles was still considerable. To overcome this difficulty it was 

 decided to construct astrolabes with very large circles, and the instrument 

 carried by Vasco da Gania in his famous voyage round the Cape in 1497 had a 

 circle which measured just over two feet in diameter. The size of the instru- 

 ment certainly made it unwieldy, and so it was necessary to suspend it from 

 some sort of stand, which meant that it could not have been used with much 

 success on board ship. Vasco da Gama seems to have been fully alive to this, 

 and so we find him, when he arrived at St. Helena Bay, not far from the Cape, 

 bringing his instrument on shore and fitting it up on a stand. His observation 

 and method of obtaining the latitude of this spot is of considerable interest, and 

 may perhaps be taken as a fair example of the kind of work that was then 

 done 



The sun's meridian altitude measured was 76° 20', which gave a zenith 

 distance of 13° 4(y. The declination found from the tables was 19° 21' S., so 

 by adding this to the zenith distance the resulting latitude was 33° 0' S. I 

 have recently tried to find out how near this was to the true latitude, but it 

 seems to be difficult to say exactly where the instrument was erected. If we 

 take the head of the bay as the spot, the error is apparently 13', since the latest 

 Admiralty chart gives 32° 47' ^S. This error appears to be somewhat larger than 

 might have been expected, but still, taking all things into consideration, it was 

 not so bad after all. I have on several occasions made altitude observations 

 with rough home-made instruments of the astrolabe type, to see what could 

 reasonably be expected, and have found that with care it is possible to get a 

 latitude with an error not exceeding 5' to 7', taking a mean of several readings. 



The diflficulty of taking anything like accurate observations at sea was for 

 centuries a very serious one, and long before the invention of the reflecting 

 quadrant or sextant many were the attempts to devise some instrument for 

 accomplishing this. 



Next to the astrolabe, and various forms of quadrants with a sighting 

 arrangement and plumb-bob, the old cross-staff came into use. This consisted 

 of two rods_ or pieces of wood at right angles to each other. The shorter piece 

 had a hole in the centre, and was made to slide along the other. The eye was 

 placed at the end of the long piece, and the sliding piece or cross moved along 

 until one end of it cut the sea horizon and the other the sun. The altitude was 

 then read off on the long staff, whicli was graduated for the purpose. This was 

 es.sentially a .seaman's instrument, and was in common use about 3(X) years ago— 

 in fact, until the famous old Arctic explorer, Capt. John Davis, of the sixteeiith 

 century, improved upon it by bringing out his ' Back-staff,' which enabled a 

 man to take altitudes with his back to the sun instead of half blinding himself 

 by looking straight at it. 



With instruments such as these only the roughest measures could be obtained, 

 and it was not until the ingenious invention of the reflecting octant, suggested 

 first_ of all by Sir Isaac Newton, that anything approachin.g accuracy was 

 possible. Hadley's quadrant was the first of such instruments to be put into 

 a,ctual use, but there is no doubt that the idea should be ascribed to the famous 

 Sir Isaac Newton, although the instrument was probably independently invented 

 by Had ley. 



With the invention of the sextant, or its predecessors the octant and quad- 

 rant, rapid progress was made in improvements in navigation and surveying 

 instruments. 



The introduction of the Nonius by Peter Nunez in the middle of the sixteenth 

 century, and later of the Vernier by the Frenchman Francis Vernier, which, 



