Decemiseu 7. 1.S83. 1 



science:. 



731 



depression is i)owerless to draw it towards the 

 centre. Only when its velocitj- is decreased by 

 friction does tlie barometric gradient, just be- 

 fore produced by the centrifugal force, urge 

 the wind inwards to the middle of the storm. 

 The additional gradient, therefore, represents 

 potential encrg\', derived from the actual 

 energy of the rotating winds, and all ready 

 to be transformed into actual energy again, 

 as soon as friction has destroyed some of the 

 velocity of rotation. 



The general interaction of the storm-forces 

 may now be thus summarized : in obedience 

 to a centripetal tendency-, produced by ditfer- 

 ences of temperature or of pressure, or both, 

 the air moves along the surface to the region of 

 low pressure. On its way, the deflective force 

 ai-ising from the earth's rotation turns it con- 

 tinually to one side, and so gives it a more 

 and more nearly circular path : and, in addition 

 to this, its rotary velocity increases as much as 

 its radius of rotation decreases : the tangen- 

 tial component of its spiral motion must there- 

 fore continually increase. "With tlie increase of 

 this component, and the decrease of the radius 

 of rotation, the centrifugal force (v--i-r) must 

 increase rapidly, and soon come to equal and 

 counterbalance the original centripetal force, 

 and at the same time greatly increase the 

 barometric gradients. At this point the wind 

 would blow in a circular path, were it not tliat 

 friction with the sea or ground is continually 

 consuming some of its velocity, and thus de- 

 creasing its centrifugal force, and allowing the 

 potential energy of the steep barometric gradi- 

 ent to produce centripetal motion. This de- 

 creases its radius, and at once gives it new life, 

 again to be partly destroyed and renewed as 

 before. Absolutely circular motion can there- 

 fore never be attained, although it is approached 

 very closely near the centre. At sea, where 

 friction is small, and in tropical latitudes, 

 where the strength of the storm is great, the 

 wind is unable to reacli tiie storm-centre ; for, 

 when the distance from the centre is reduced 

 to only five or ten miles, the centrifugal force 

 is so 4;reat, and the wind's course is so nearh' 

 circular, that it is carried aloft bv the up-draught 

 before it can enter noticeablj- farther : the cen- 

 tral area is therefore left unprovided with vio- 

 lent winds, and is generally a comparative 

 calm, known as the 'eye of the storm,' of 

 which there will be more to say later. The 

 general form of the storm- wind's spiral can 

 be deduced from the preceding considerations. 

 The angle between the tangential component 

 and the actual path of the wind, which is called 

 the inclination (fig. 11.). will vary with the 



relation of the circular and centripetal elements 

 of the wind'.s motion ; the tangent of the incli- 

 nation will equal the radial divided by the tan- 

 gential component : hence in the outer part 

 of the storm the inclination will be large, and 

 the wind will blow almost directly toward the 

 storm-centre ; but nearer the centre the incli- 

 nation will become smaller and smaller, and 

 the wind will blow in a more and more nearlj' 

 circular path. It will also be understood, that 

 the upper winds, less influenced by friction, 

 will near the centre have a greater velocity 

 and a less inclination than the lower ones. 

 Jloreover, the inward gradient which they pro- 

 duce will bo effective and important in urging 

 along the slowei- surface-winds, in a manner 

 better illustrated in a tornado, where this action 

 will be more fully described. 



( To be continued.) 



ON THE DEVELOPMENT OF TEETH IN 

 THE LAMPREY. 



The teeth in the myxinoid fishes are quite 

 diflierent from those of other vertebrates, and 

 have hitherto been supposed to belong in an 

 entirely different category. Nothing has been 

 known with regai'd to their development, except 

 a brief statement as to their mode of succes- 

 sion in Petromyzon b}- Professor Owen, in his 

 'Odontography.' 



The teeth of the lamprey are horny, and of 

 simple conical shape, disposed concentrically in 

 the dome-shaped mouth. Besides tliese, there 

 are horny lingual aud p.alatal teeth. 



The kindness of m\- friend Professor Bcnecke 

 of Konigsberg, who sent me a number of lam- 

 preys at the end of their metamorphosis from 

 Ammocoetos, has enabled me to follow out the 

 development of these horny teetli with unex- 

 pected results ; for, as far as the essential part 

 of the process is concerned, it differs but slight- 

 ly from the normal course of true dental devel- 

 opment. There is first formed a low conical 

 papilla of somewhat reticulate tissue, belonging 

 to the mesoblast {m.p.), and continuous with 

 the dermis, which in this, as in other verte- 

 brate.'?, is of mesoblastic origin. Over this 

 papilla the epil)last which lines the cavity of 

 the mouth becomes extremely thick, and con- 

 sists of very numerous layers of cells. All of 

 these layers can bo continuouslv traced into 

 the other epiblast of the mouth, as well as that 

 of the external skin. In the stage here figured 

 there ma3- be seen, immediately overlying the 

 mesoblastic papilla, a layer of epiblastic cells 

 irregularly columnar and poh'gonal in shape 

 (I'.o.). These cells are the homologuc of the 



