544 



any such should be avoided, and, as far as possible, the compass 

 should be so placed that they may act as correctors of the general 

 magnetism of the hull. 



As most compasses are affected by the magnetism of the ship to 

 an amount depending on their elevation, and the direction of the ship 

 in building, the disturbances will be large comparatively, except in 

 those vessels built head east or west. 



A series of Tables is appended, wherein the magnetic coefficients 

 and ship's force and direction of the various classes of vessels are 

 given, the ships being classed according to the nature of their mate- 

 rial and machinery. 



XII. " On the Sources of the Nitrogen of Vegetation ; with 

 special reference to the Question whether Plants assimilate 

 free or uncombined Nitrogen." By J. B. LAWES, Esq., 

 F.R.S.; J. H. GILBERT, Ph.D., F.R.S. ; and EVAN PUGH, 

 Ph.D., F.C.S. Received June 21, 1860. 



(Abstract.) 



After referring to the earlier history of the subject, and especially 

 to the conclusion of Saussure, that plants derive their nitrogen from 

 the nitrogenous compounds of the soil and the small amount of 

 ammonia which he found to exist in the atmosphere, the Authors 

 preface the discussion of their own experiments on the sources of the 

 nitrogen of plants, by a consideration of the most prominent facts 

 established by their own investigations concerning the amount of 

 nitrogen yielded by different crops over a given area of land, and of 

 the relation of these to certain measured, or known sources of it. 



On growing the same crop year after year on the same land, with- 

 out any supply of nitrogen by manure, it was found that wheat, over 

 a period of 14 years, had given rather more than 30 Ibs. barley, 

 over a period of 6 years, somewhat less meadow-hay, over a period 

 of 3 years, nearly 40 Ibs. and beans, over 1 1 years, rather more than 

 50 Ibs. of nitrogen, per acre, per annum. Clover, another leguminous 

 crop, grown in 3 out of 4 consecutive years, had given an average of 

 1 20 Ibs. Turnips, over 8 consecutive years, had yielded about 45 Ibs. 



The graminaceous crops had not, during the period referred to, 



