z 3 o SCIENTIFIC APPARATUS. 



star ; and in many of the early voyages of discovery both astro- 

 labe and cross-staff were' in use. But the cross-staff was 

 generally preferred by our English mariners because the graduation 

 was larger and more easily read off. It was a very simple instru- 

 ment, consisting of a graduated pole, with cross pieces, called 

 transversaries (of which there were four, used according to the 

 altitude), also graduated, which were fitted to work on it. The 

 bearing of the sun was taken by compass, to ascertain when it 

 was near the meridian, then the end of the long staff was placed 

 close to the observer's eye, and the transversary moved until one 

 end exactly touched the horizon and the other the sun's centre. 

 This was continued until the sun dipped, when the meridian 

 altitude was obtained. The back-staff was an improvement on 

 the cross-staff, invented by the great arctic navigator, John 

 Davis. It was fitted with a reflector, and it was thus the first 

 rough idea of the principle of the quadrant and sextant. The 

 astrolabe was a metal circle graduated round the edge, with a 

 limb, called the alhidada, fixed to a pin in the centre, and working 

 round the graduated circle. It had two sights fitted upon it, 

 one at each end. The instrument was suspended by a ring, so 

 as to hang vertically on one hand, while with the other the 

 alhidada was worked up and down until the sun could be seen 

 through both the sights. It then gave the zenith distance. The 

 cross-staff was used for low altitudes because both ends of the 

 transversary could easily be seen at the same time ; and the astro- 

 labe for high altitudes. With the invention of these instruments 

 came instructions for their use, and for working out observations. 

 In this country, the first of these was " The old Rutter of the 

 Sea," printed in 1490. Then followed the " Seaman's Secrets," 

 by John Davis, and " A Regiment of the Sea, containing very 

 necessary matters, with a perfect sea carde," by Thomas Hood, 

 published in 1596. These ancient manuals contained definitions, 

 a treatise on the use of the sea card, or chart and compass, tables 

 of declination, instructions for observing with cross-staff and 



