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LVII. A Description of a nexo Patent Instniment, or Celestial 

 Compass, adapted for ascertaining the Deviation of the Mag- 

 netic Needle, by simple Inspection, in any Part of the World ; 

 for finding the Latitude when the Horizon is obscured ,- and 

 for steering Ships without Magnetic Aid. Invented by George 

 Graydon, Captain in the Corps of Royal Engineers.* 



General Description of Plate I. 

 DLATE I. represents the celestial compass mounted in 

 •*- gimbals, as a means of detecting by simple inspection, as 

 long as any of the heavenly bodies are visible, the changes to 

 which the magnetic needle is exposed in all parts of the world, 

 in consequence of the local magnetism of the earthf or me- 

 teoric influence, independent of the local attraction of the iron 

 of the ship, or the annual change of the variation of the needle. 

 It serves also as a substitute for the magnetic compass in high 

 northern or southern latitudes, where the directive power of the 

 magnetic compass becomes almost useless from its feeble and 

 uncertain action. 



In many cases it is impracticable to calculate the variation, 

 in consequence of the state of the sky or atmosphere preclu- 

 ding the observation for an amplitude or azimuth ; the former 

 can only be taken at the rising or setting, and the latter can 

 only be taken with accuracy when the sun or star is at low 

 altitudes. It will frequently happen, and particular]^ in the 

 channels of the British isles, that the horizon, and several de- 

 grees above it, is so obscured for some days together, that these 

 observations cannot be made, although the sky is sufficiently 

 clear at a greater altitude. In these circumstances no varia- 

 tion can be calculated with accuracy for those days, except from 

 the table of variation, which is now becoming every day more 

 and moi-e uncertain. It is also to be observed, that, often, 

 the detection of those accidents above mentioned (indeed al- 

 most always) depends on the observations, and may remain 

 imperceptibly operating for a length of time, during which no 

 observations to correct the variation can be had. 



Plate I. also represents the celestial compass adapted for 

 ascertaining the latitude when the horizon is obscured, or is 

 rendered uncertain by refraction. 



Description of Plate I. of the Celestial Compass as adapted 



for detecting by simple Inspection, as long as any one of the 



Heavenly Bodies is visible, the Deviation of the Magnetic 



Needle. {See Plate I.) 



A B represents the face or dial plate of the instrument ; this 



* Tlie Celestial Compass may be procm-ed at Messrs. Warre and Brothers, 

 13, Austin Friars. 



•f- Instances of the powerful effect which the local magnetism of the earth 

 has upon the magnetic needle, arc given in A|ipcndix No. II. 



plate 



