XIV 



length, placed near each other on the card). The results of the observa- 

 tions and experiments, reduced by aid of Napier's graphic method, and 

 subjected to a thorough harmonic analysis, are described in a joint paper 

 by Smith and Evans, published in the Transactions of the Royal Society 

 for 1861. They show, in the expression for the deviation, sextantal and 

 octantai terms* very large in the case of the ' Great Eastern,' and com- 

 paratively small when the Admiralty standard compass was tested in cir- 

 cumstances otherwise similar. Whether single needles or double needles 

 were used, it was found that the smaller the needle the smaller were the 

 sextantal and octantai terms. Single needles gave greater terms of this 

 class than double needles of the same magnitude, arranged as in the 

 Admiralty compass. 



The merit of giving almost evanescent sextantal and octantai terms, 



of the highest name. Reversion to the old Chinese species, with single needle less than 

 an inch long and unloaded by a compass card, would be an improvement on the pre- 

 sent ordinary usage of first-class ocean steamers. 



The direction of part of the reactionary improvement required is clearly pointed out in 

 the following note on the comparative merits of large and small compasses, extracted from 

 Captain Evans's ' Elementary Manual for the Deviation of the Compass in Iron Ships :' 

 " Of late years much diversity in practice has prevailed as to the size of compasses for 

 " use on board ship. The Admiralty Standard card of 7 inches diameter, for example, 

 "is fitted with needles the maximum lengths of which are 7 inches, while in large 

 " passenger steam-vessels the needles are frequently 12 to 15 inches, and even longer. 

 " The chief object in the employment of large compasses is to enable the helmsmen 

 "to steer to degrees ; and a more accurate course is thus presumed to be preserved." 



" With reference to this increased size, it must be observed that competent authorities 

 " limit the length of efficient compass needles to 5 or 6 inches ; beyond this limit an 

 " increase of length is alone accompanied by an increase of directive power in the 

 " same proportion ; and if the thickness of the needle be preserved, the weight, and 

 " consequently the friction, increase in the same ratio. No advantage of directive 

 " power is therefore gained by increase in length ; but with the increased weight of the 

 " card and appendages the increase of friction probably far exceeds the increase of direc- 

 " tive force : sluggishness is the result, which is further exaggerated by the extreme slow- 

 " ness of oscillation of long needles compared with short needles." 



" Large cards, however convenient in practice, are therefore not without danger ; for 

 " the course steered may deceive the seamen by seeming right to the fraction of a 

 " degree, but which avails little if the card is wrong half a point, and the ship in con- 

 " sequence hazarded. In the opinion of the writer the present Admiralty standard 

 " card is as large as should be used for the purposes of navigation, and that, as regards 

 " safety in the long, steady, and fast ship, the choice is really between the Admiralty 

 " card and a smaller one. In short the case may be thus stated : the smaller a card 

 " the more correctly it points ; the larger a card the more accurately it can be read." 



When the needles of a standard compass are reduced to something like half an inch 

 in length, and not till then, will the theoretical perfection and beauty, and the great 

 practical merit, of Airy's correction of the compass by soft iron and permanent magnets 

 (which theoretically assumes the length of the needle to be infinitely small in propor- 

 tion to its distance from the nearest iron or steel) be universally recognized and have 

 full justice done to it in practice. 



* That is to say, terms consisting of coefficients multiplying the sines or cosines of 

 six times and of eight times the ship's magnetic azimuth. 



