TORNADO. 



TORSION. 



286 



in forests, bogs, and heaths. [PoTENTltLA.] The root, or rather the 

 rhizoma, is officinal. As the plant flowers in June and July, the best 

 time to collect the rhizomata is in April and May. Those gathered in 

 autumn, while they remain moist, are phosphorescent. The roots of 

 the Tormentilla (Potentilla) reptans (Linn.), of the Potentilla Com- 

 inarum, those of the common strawberry, of the Qeum urbanum, and 

 also of the Polygonum bistorta, are frequently confounded with those 

 of the true tormentU errors of no great importance aa far as their 

 medical employment is concerned, as they possess properties similar in 

 kind, but inferior in degree. In Italy the root of the Geranium striatum 

 is substituted for it. 



The rhizomata of the genuine plant are large in proportion to the 

 branches they bear. They lie obliquely in the earth; old ones are 

 knotty or resemble knobs, from 1J to 2 inches thick; younger ones 

 are cylindrical, irregularly branched, the branches 1 to 2 inches long, 

 and from one-fourth to one-half inch thick, curved and twisted. The 

 epidermis and liber are very thin, but firm. The central part presents 

 two or more concentric circles. The colour of the interior, when 

 fresh, is a rose-red or fleshy colour ; but when dried, it inclines more 

 to a reddish or brownish yellow ; in very old specimens it becomes 

 white. It can be easily powdered : the powder is of a bright brownish- 

 red. The rose-odour of the fresh root is lost by drying. Taste purely 

 and strongly astringent Specimens which are dark externally, and 

 woody and white within, are to be rejected. 



Water distilled from the fresh root has an agreeable rose-like odour. 

 This plant contains more tannin than any other, except catechu and 

 galls. Tormentil is the most powerful of our indigenous astringents, 

 and more easily assimilated than oak-bark or galls. Though improper 

 in active hemorrhages, in passive discharges it is very useful, and may 

 be given with aromatics, or opiates, or chalk, as in the compound 

 powder of chalk. Few medicines are more efficacious for drying up 

 the slimy mucus in which worms nestle in the intestines of children, 

 than the compound powder of chalk. Infusion made with cold water 

 is preferable to the decoction. The extract made in the common way 

 soon spoils. But valuable as this substance is in medicine, it is of still 

 greater utility in the arts and in agriculture. It may be most bene- 

 ficially employed to tan leather, both where the oak grows and where 

 it is absent, since one pound and a half of powdered tormentil is equal 

 in strength to seven pounds of tan. It is used in Lapland and the 

 Orkney Isles, both to tan and to dye leather, and in the latter parts to 

 dye wonted yarn. By long boiling the tannin is converted into gum, 

 and in times of scarcity the poor may collect and obtain much nourish- 

 ment from the root. But the great service this plant renders in 

 husbandry is its chief merit. Where it grows abundantly in wet 

 pastures, the rot in the sheep is unknown. Where the heather has 

 been burned on the Highland hills, this plant springs up spontaneously 

 with the tender grass. [AXTHELMINTICS.] 



TORNADO. [WHIRLWIND.] 



TORRICELLIAN VACUUM. [BAROMETEU.] 



TORRICELLI'S THEOREM. [HYDRODYNAMICS.] 



TORRID ZONE. [Zost] 



TORSION is that force with which a thread or wire returns to a 

 state of rest when it has been twisted by being turned round on its 

 axis : the thread or wire, which is suspended vertically, is attached at 

 the upper extremity to some object, and at the lower extremity is a 



c 



weight with a horizontal index, or a stirrup, which is to carry a needle 

 or bar in a horizontal position. 



or bar in a ozontal position. 



Let Y be the wire, w the weight or stirrup, and A B an index 

 needle, and let I a c be part of a graduated ring on the same level 



the needle ; then, on turning the object w round till a mark on the 

 extremity A of the index is brought to any point, b, on the ring, the 

 wire becomes twisted ; and when the power by which w is turned is 

 removed, the elasticity of the wire causes the point at A to oscillate 

 within the ring through an arc, as b a c, which continually diminishes 

 till the index rests in its original position. 



Under ELASTICITY is given an investigation from which it is proved 

 that, while the force of torsion is moderate, its intensity is directly 

 proportional to the angle or arc through which the extremity A of the 

 index is moved in twisting the wire. It is also proved that T, the time 

 of a complete oscillation, is constant, or that the vibrations are isochro- 

 nous, like those of a pendulum which is acted upon by gravity ; and 

 further, that when a body, as w, is suspended, the squares of the times 

 of vibration vary directly as the momentum of the body's inertia, and 

 inversely as the force of torsion : consequently when the forms and 

 weights of suspended bodies are the same, the force of torsion varies 

 inversely with the square of the time. With respect to the efl'ecta 

 which a variation in the length of the wire will cause in the force of 

 torsion, it may be observed that in proportion as the lengths of the 

 wires are increased, points at the lower extremities must be turned, 

 about the axis, through greater arcs, in order to produce equal degrees 

 of torsion at equal distances from the points of suspension ; and hence, 

 if the number of revolutions be equal, the force of torsion will be in- 

 versely proportional to the length of the wire : it follows therefore that 

 the time of a vibration varies directly with the square-root of the length 

 of the wire. 



These deductions from theory are confirmed by their agreement with 

 the results of the numerous experiments made by M. Coulomb with an 

 apparatus similar to that which is represented above; the times hi 

 which a certain number of isochronous vibrations were made with wires 

 of different lengths, and carrying at their lower extremities cylinders 

 of different weights, being observed. By comparisons also of experi- 

 ments on wires of the same length and of different diameters, conse- 

 quently of different weights, Coulomb found that the times of vibration 

 were inversely proportional to the weights, or to the squares of the 

 diameters of the wires ; and since the force of torsion varies inversely 

 with the squares of the times, it follows that when the wires are of the 

 like material and of equal lengths, the force of torsion varies directly 

 with the fourth power of the diameter. M. Poisson, in a memoir on the 

 equilibrium and movement of elastic bodies (' Memoires de 1'Acaddmie 

 des Sciences,' torn, viii.), has deduced the same law from purely theo- 

 retical considerations. 



It may be convenient to compre the force of torsion with that of 

 gravity, and for this purpose it will be necessary to observe merely 

 that the time in which a pendulum, whose length is I, makes a 



complete oscillation in a very small arc ia expressed by ir f _ \* 



^PENDULUM], where y represents the force of 'gravity : then the time 

 m which the torsion wire vibrates once on its axis being made equal to 

 the time in which a simple pendulum vibrates, we have (using the 



K S M I 



formula in ELASTICITY), = ; therefore, as the momentum of 



<J 



inertia for a torsion wire suspending a body of a given form can be 

 computed, and as I may be found from the observed time of a vibra- 

 tion, the value of (the coefficient of the force of torsion) can be 

 ascertained from this equation. 



The torsion of slender wires was first employed by Coulomb for the 

 purpose of determining the intensities of forces hi nature and the laws 

 of their action in circumstances which render direct methods in- 

 applicable : his experiments were performed with an instrument 

 which he invented, and which he designated a toriion balance. Under 

 ELECTROMETER is given a description of the instrument and of the 

 method of employing it in finding the laws of electric attractions and 

 repulsions ; and it will, therefore, be sufficient in this place to explain 

 its application in determining those of magnetic action. For thia 

 purpose Coulomb adapted to the suspending wire, which was of copper, 

 a small stirrup, as w, also of copper, in which could be placed a mag- 

 netised needle of steel. Before this was done, however, a copper needle, 

 equal in weight to the magnetised needle which was to be \ised in the 

 experiment, was placed in the stirrup, and the plate D at the top of the 

 glass case was turned round till one extremity of the copper needle, 

 which turned with the plate,} was brought to the zero of the gradua- 

 tions on the horizontal circle 6 a c hi the case, the suspending wire 

 being in an untwisted state : the whole case was afterwards turned 

 round till the needle, still pointing to zero, was in the direction of the 

 magnetic meridian, which had been previously determined. The 

 copper needle was then taken away, and the magnetised needle put hi 

 the Btirrup ; and as soon as it was at rest in the magnetic meridian, the 

 suspending wire was twisted by turning the stem E, to which it is 

 attached at the upper extremity of the case, till the index there had 

 passed over some given nuin,ber of degrees, which in one experiment 

 was 860*. The suspended needle was thus made to deviate from its 

 previous position 104 degrees, in which state the horizontal force of 

 terrestrial magnetism was in equilibrio with the force of torsion ; and 

 the angle of torsion was then equal to 3494 ( = 360 10J). On 

 turning the index at E through two revolutions, the needle was observed 

 to rest between the opposing forces, at 21{ from its original place, 



