MAGNETISM AND THE COMPASS. 661 



General Remarks on the Nimbus. — Any of the modifications just de-^ 

 scribed may increase so much as to obscure the sky, without ending in 

 rain ; before which, the pecuhar characteristic of the rain-cloud may 

 always be distinguished. In order to get a clear idea of its formation, you 

 may observe a distinct shower in profile, from its formation to its fall in 

 rain. You may then observe Cumulus first arrested, then Cirro-stratus or 

 Cirrus may appear to alight on its top ; the change to Cumulo-stratus then 

 goes on rapidly, and this Cloud, increasing in density, assumes that black 

 and threatening appearance known as an indication of rain. Presently 

 this blackness is changed to a grey obscurity, and this is the criterion of 

 the actual formation of water, which now begins to fall, and constitutes 

 the cloud a Nimbus, while a Cirriform crown of fibres extends from the 

 upper part of the clouds, and small Cumuli enter into the lower part. 

 After the shower has spent itself, the Cloud resumes its title of Cumulo- 

 stratus, and thence probably changes into a different modification ; and if 

 -X/umulo-stratus appears again, it indicates a return to rain. 



7.— ON MAGNETISM AND THE COMPASS. 



Concerning the history of the magnetic needle, we have many and 

 vague notices of its high antiquity. It is mentioned by Homer and 

 Aristotle, and by many subsequent classical writers. To come to more 

 modern times, it was known, in Europe, at the time of the Crusades, 

 in about a.d. 1150, and it is very probable that the knowledge was 

 derived from the Arabians, during those expeditions. The Chinese were 

 acquainted with it ages before this. We are told by the Jesuit mis- 

 sionary, Du Halde, that the Chinese Emperor, Hoang-ti, possessed an 

 instrument which pointed to the South, so early as the year 2634 b.c, or 

 over 4,500 years ago ; the same author gives subsequent notices of the 

 Compass in China, proving its very great antiquity among that people. 



It has usually been considered that Columbus, in his voyage from 

 Portugal, on the discovery of America, first observed the Variation of the 

 needle from the true North. But it is not improbable that the Variation 

 was discovered nearly 200 years before Columbus made this change known, 

 as it is mentioned in one of the earliest treatises on Magnetism, by Peter 

 Adsiger, in 1269 ; the authenticity of this, however, is doubted by some. 

 The wonderful property of the Dip of the needle was first observed by our 

 countryman, Eobert Norman, a maker of compasses, in 1576. 



Magnetism is a principle which is evidently allied to, if it is not identical 

 with. Electricity and Galvanism. For, in causing any or either of 

 these principles to become evident to our senses, we produce, at the same 

 time, the others ; and it may be here stated, that five apparently dis- 

 similar effects are inevitably caused in the production of either, viz., 

 Light, Heat, Chemical Action, Electricity, and Magnetism. By the pro- 

 duction of Light we cause Heat and Chemical Affinity, and these will also 

 produce Electricity, and will cause the magnetic needle to swerve from the 



