152 PHYSICAL GEOGEAPHY OF THE SEA, AND ITS I^IETEOEOLOGY. 



time-piece, the hour of the day may be told by these maxima and 

 minima, each group of which occurs twice a day and at six-hour 

 intervals. These invisible ebbings and flomngs — the diurnal change 

 in the electrical t^sion — the diurnal variation of the needle, — and 

 the dim-nal rising and falling of the barometer, — follow each other as 

 closely and as surely, if not quite as regularly, as night the day. 

 Any cause which produces changes in atmospheric pressure invaria- 

 bly puts it in motion, giving rise to gentle airs or fmious gales, 

 according to degree ; and here, at least, we have a relation between 

 the movements in the air and the movements of the needle so close 

 that it is difficult to say which is cause, which effect, or whether 

 the two be not the effects of a common cause. 



349. Indeed, such is the natm^e of this imponderable called mag- 

 The question raised by nctism, and sucli the suggostious made by Faraday's 

 modern researches, discovcries, that the qucstiou has been raised in the 

 minds of the most profound philosophers of the age whether the 

 various forces of light, heat, and gravitation, of chemical affinity, 

 electricity, and magnetism, may not yet be all traced to one 

 common som^ce. Sm'ely, then, it cannot be considered as miphilo-, 

 sophical to inquire of magnetism for some of the anomalous move- 

 ments that are observed in the atmosphere. These anomalies are 

 many ; they are not con&ied to the easting of the trade-v^ds ; 

 they are to be fomid in the counter- trades and the calm belts also. 

 There is reason to believe, as has already been stated (§ 288), that 

 there is a crossing of the winds at the calm belts (§212), and it 

 was promised to go more into detail concerning the ch^cumstances 

 which seem to favom- this belief. Our researches have enabled us, 

 for instance, to trace fi'om the belt of calms, near the tropic of 

 Cancer, which extends entirely across the seas, an efflux of ah' both 

 to the north and to the south. From the south side of this belt 

 the air flows in a steady breeze, called the north-east trade-winds, 

 towards the equator (Plate I.) ; on the north side of it, the prevail- 

 ing winds come from it also, but they go towards the north-east. 

 They are the well-known westerly winds w^hich prevail along the 

 route from this coimtry to England in the ratio of two to one. 

 But why should we suppose a crossing to take place here ? We 

 suppose so from these facts : because throughout Em'ope, — the 

 land upon which these westerly winds blow, — precipitation is in ex- 

 cess of evaporation, and because at sea they are going from a warmer 

 to a colder climate ; and therefore it may be inferred that nature 

 .exacts fi-'om them what v,^e know she exacts from the air under 



