^ Analyses of Books. [Jax. 



the application of the doctrine of probabilities to moral and poli- 

 tical observations, he enters upon its application to experimental, 

 observations, and shows that when we have ascertained the mean 

 error, that will amount nearly to our deviation from accuracy. 

 But he observes, that in experiments so many things are of 

 necessity left out of view that the application of the doctrine of 

 probabilities to them is not likely to lead to any advantageous 

 result. 



The second section is on the mean density of the earth. He 

 shows, as had been already done by Laplace, that the pressure 

 of the strata composing the earth is fully sufficient to account 

 for the oreater density of the central parts above the superficial,, 

 without supposing the materials in the centre to be different from 

 those at the surface. 



The third section is an investigation of the effects of the irre- 

 oularities at the earth's surface upon the plumb line, and on 

 gravitation. In the fourth section, he shows that Laplace's 

 theorem for the length of the convertible pendulum rolling on 

 -equal cylinders, may be deduced from an elegant investigation 

 of Euler in the ]yova' Acta Petropohtana for 1788, p. 145. 



VL On the Anomahj in the Variation of the Magnetic Needle 

 as observed on Ship-board. By William Scoresby, Jun. Esq. 



Mr. Scoresby gives a table of magnetical observations which 

 he made on board the Esk, in his voyage to the Greenland 

 whale fishery in 1817. From these observations, and from the 

 lucid remarks of Capt. Flinders, he deduces a niunber of infer- 

 ences, some of the most important of which are the following: 



1. Such parts of the iron employed in the construction of 

 ships as have a perpendicular position are magnets, the upper 

 ends being south, the lov.er north poles in our hemisphere, and 

 the contrary in the southern hemisphere. 



2. The combined influence of all this iron seems to be concen- 

 trated into a kind of magnetic focus of attraction, the principal 

 south pole of which he conceives to be situated near the middle 

 upper deck, but nearer the stem than the stern. 



3. This focus of attraction so influences the compass needle 

 that it is subject to an anomaly or variation from the true meri- 

 dian, different from what is observed by a compass on shore ; 

 the north point of the compass being constantly drawn towards 

 the focus in our hemisphere, and the south point in the opposite 

 hemisphere. 



Mr. Scoresby, sen. observed 20 years ago, that a ship beating 

 to the northvi-ard with a north v.'ind appeared to lie nearer the 

 wind than when beating to the southward with a southerly wind. 

 This he ascribed to the attraction of the ship upon the compass. 

 On this account, the author has been in the habit of allowing only 

 2 or 2± points' variation on the passage outward to Greenland 

 with a northerly or north-easterly course, but generally thre« 



