358 DISCOVERIES OF ROSS, 1818. 



information obtained on every interesting subject 

 in high northern latitudes which are rarely visited 

 by scientific men. 



Among other important objects, which the occa- 

 sion will present, is that of determining the length 

 of the pendulum vibrating seconds in a high de- 

 gree of latitude. For this purpose each expedition 

 is supplied with a clock having a pendulum cast in 

 one solid mass, vibrating on a blunt knife-edge 

 resting in longitudinal sections of hollow cyhnders 

 of agate ; and to each clock is added a transit in- 

 strument. Each ship is also supplied wath the 

 following instruments — a dipping needle on a new 

 construction which, at the same time, is calculated 

 to measure the magnetic force — an azimuth com- 

 pass improved by Captain Kater — a repeating 

 circle for taking terrestrial angles — an instrument 

 for ascertaining the altitude of celestial bodies 

 when the horizon is obscured by fogs, which is 

 almost always the case in high latitude's — a dip- 

 micrometer and dip-sectcr, invented by Doctor 

 Wollaston, to correct the variation of the real dip 

 from that given in the tables, arising principally 

 from the difference between the temperature of the 

 sea and the atmosphere — a macrometer, also in- 

 vented by Doctor Wollaston, for measuring di- 

 rectly the distance of inaccessible objects, by 

 means of two reflectors, mounted as in a common 

 sextant, but at a greater distance from each other 

 — three chronometers to each ship — ^hydrometer, 



