332 



NATURE 



[Fed. 21, 1878 



cap, amounts in all to about five grains. It need not be more for 

 a 24-inch than for a lo-inch compass. For the lo-inch compass 

 the whole weight on the iridium point, including rim, card, silk 

 threads, central boss, and needles, is about 180 grains. The 

 limit to the diameter of the card depends upon the qumtity of 

 soft iron that can be introduced without inconvenient cumbrous- 

 ness on the two sides of the binnacle to correct the quad- 



FlG. I. 



rantal error. If, as sometimes may be advisable in the case 

 of a pole or masthead compass, it be determined to leave the 

 quadrantal error uncorrected, the diameter of the compass-card 

 may be anything from 12 to 24 inches, according to circumstances. 

 A 24-inch card on the new plan will undoubtedly have less fric- 

 tional error or *' sluggishness " for the same degree of steadiness 



y%?%i^>^^^-->^^-^-- 



■:'::.^^.^;f<:<0-<:''''^;i^<^^^>^fim^ 



Fig. a. 



than any smaller size ; but a 12-ittch card works well even in 

 very unfavourable circumstances, and it will rarely, if ever, be 

 necessary to choose a larger size unless for convenience to the 

 steersman for seeing the divisions, whether points or degrees. 

 You see hanging over the table, from the roof, one of my 12 -inch 

 I oIe-.ompass;s. Specimens of i5-incji and 24-inch pole-com- 



passes have also been made. The last-mentioned may be looked 

 at with some curiosity, as being probably the largest compass in 

 the world. It will no doubt be properly condemned as too 

 cumbrous for use at sea, even in the largest ship, but there caa 

 be no doubt it would work well in a position in which a smaller 

 compass would be caused to oscillate very wildly by the motion 

 of the ship. The period of the new loinch compass is in this 

 part of the world about forty seconds, 

 which is more than double the period of 

 the A card of the Admiralty standard 

 compass, and is considerably longer than 

 that of the ordinary lo-inch compass, so 

 much in use in merchant steamers. The 

 new compass ought, therefore, according to 

 theory, to be considerably steadier in a 

 heavy sea than either the Admiralty com- 

 pass or the ordinary lo-mch compass, and 

 actual experience at sea has thoroughly 

 fulfilled this promise. It has also proved 

 very satisfactory in respect to frictional 

 error ; so much so that variations of a 

 steamer's course of less than half a degree 

 are shown instantly and surely, even if the 

 engine be stopped, and the water perfectly 

 smooth. 



With the small needles of the new com- 

 pass, the complete practical application of 

 the Astronomer-Royal's principles of cor- 

 rection is easy and sure : that is to say, 

 correctors can be applied so that the com- 

 pass shall point correctly on all points, and 

 these correctors can be easily and surely 

 adjusted at sea, from time to time, so as 

 to correct the snallest discoverable error 

 growing up, whether through change of the 

 ship's magnetism, or of the magnetism 

 induced by the earth, according to the 

 changing position of the ship. To correct 

 the quadrantal error I use a pair of solid 

 or hollow iron globes placed on proper 

 supports, attached to the binnacle on two 

 sides of the compass. This mode is pre- 

 ferable to the usual chain boxes, because a 

 continuous globe or spherical shell of iron 

 is more regular in its effect than a heap of 

 chain, and because a considerably less bulk 

 of the continuous iron suffices to correct 

 the same error. When in a first adjustment in a new ship, or 

 in a new position of a compass in an old ship, the quadr-.nt d 



Fig. 



Fig 



error has been found from observation, by the ordinary practical 

 methods, it is to be corrected by placing a pair of globes in 

 proper positions according t) the follovvini]; table :— 



