�1ECHAl'\ICAL MovE�IENTS. //,._ -··------· --···--···-·-···· ···---•- •·••·------·--- -·-·-- 306. Three-l gged pendulum escapement. ; raises one of the weighted pallets out of the The pallets ar e formed in an opening in a: wheel at each vibration. vVhen the pendu­ plate attached e to th pendulum, and the ! !um r turns the pallet falls with it, and the e three teeth of the escape-wheel operate on ; weighte of the pal et gives the impu se. the upper and lower pallets alternately. i l l One tooth is shown in op ration on th ! upper p . e e allet 310. Thre -leo-o-ed00 0o-ravitv ., esca1)emcnt. Th lifting ofe the pallets, A and B, is don by eth three pins n ar the center of thee 307. A modification of the above with; escape-wheel,e th palletse vibrating from two Jong stopping te th, D and E. A and Bl centers near th epoint of suspension of th e e are th pallets. 1 pendulum. Thee escap -wheel is locked by e i means of stops, D and eE, on th pa lets. i e l I 308. A detached pendulum escapem nt, J e . le ving the p ndulum, P, free or detached j 31 1 Double three-l gged gravity escape­ a e from the escape-wh el, except at th time of ! Two locking-wheels,e A, B, C, and e e ment. receiving the impulse and unlocking the! a, b, c, are here used with one set of lifting­ whe l. here is but on pallet, I, w�1ich / pins between th m. The two wheels ar � � � _ e receiv s impulse only dunng the v1brat1ons t set wide enoucrh e apart to allow th pallets e e of the p ndulum to the lef�- The leYer, Q, i to lie b tween�hem. Th teeth of th first­ e . e e locks the escap -wheel until JUSt before the' mentioneed lockin

MECHA!'\ICAL lvfOVEllIE�TS. 79

313. Chrono n ter escape nent, the_ form ';,°wc ommonly ! ri,;ht, the c, rb-pins are made to _reduce the length ofacting '. : � _e '. constructed. .\s the balanc. rotates 111 the ,.,rect,on ofthe · p:irt of spnng. and the v1brauons of balance are made :i.rrow, the tooth, V. on the verge, pres£es th e passing- faster; and by 1noving it to the Jett a:i opposite effect is spring ag:iin�t the 1e\·er, pressing aside the le,·er ;md re- produced. mo\'in;;th e detent from the tooth of the escape-wheel. As :t b bnce returns, tooth. Y, presses aside and p:1s.;cs :,:,pri ng ; 319. Compens:i.tion ba.la!1ce. I, n, t', is the m:i.in bar of without moving lever, which then rests ng:tinst the stop, E. . balance, with timir:g screw., for rc6ulation .i.t the e::cls. t p is the only pallet upon which impu!�e i.s given. : an d t' are two compound bars, of which the ou tside is i brass and the inside stce!, carrdng weight�, b, l>'. As heat 14 LeYer chronometer escapem ent. In this thu J>allets , . : 3 · t � ' ' · mcreas es, these bars are bent mward bv� the ...... ,,·e ater expan- �nd lc\·er 1 001'. , J'kl · e 11ose of the 1e\·er escapement . . . ·:\ ' B' ' sion of the brass, and the weights are thus drawn inward, - but these pallets onlv lock th� csc:tp.. e-whe ' a - •fi: . . · . cl h ,·- d"mums · · I ung· th e - mert1a of the ,. 1J.lI :ince. As t I1e l)eat cl'1m1 ·- n« no nnpul$e. Impulse 1s ·given by teeth of escape-wheel - . i ; n1s• hes ? an opposJte « b:d:incc com• ."' a e ac e enect 1s. prod uc�<.l This ectly to .,. p ll t, C , att h

lvfECHANICAL l\.fOVEMENTS. 81

322. A very con\·e.�icnt co s ruction of l is guided by he vert cal slot in the frame, n para lel ruler for drawing, madet by cu t ng which s planedt to a truei surface. l a quadra gle through the diagonal, formingt i in·rr · · . n . . 327. 1uers from 326 111 hav1ng ro11 ers two ng - g ed triang es, A and B. It s ht �n_i l l � , substituted for the slides on he cross-head, used by shdmg the hypothenuse of one tn- • t ! sa1c• 1 ro11ers work 'mg agams· t t s ra1g· l t gUJ·d e- an"',r e upon ha of the o her. 1 fi 1 1'1 · · l t t t i bars, A, A , a tac I • d� oi_ ti1e rame. us 1s used for smalt engmest 111 France. 323. Para lel ru er consisting of a simple l straig t ruler,l B, wil h an at ached ax e, C, 328. A paralle motion inve ted by Dr. h t t l n a d pair of whee s, A, A. The wheel s, Cartwright in he year 1787. The oothed l t t whichn pro rude bu sligh ly throug the un-l whee s, C, C, have equal d ame ers a d t t t h l i t nA, der side of the ru er, ha\·e their edges numbers of eeth ; and t e cranks, A, l t n cked to take hold of the paper and keep have equal radii, and areh se in opposi e i t t the ru er always paralle with any lines directions, and consequently gh·e an equal drawn upol it. l obliqui y to he co necting-rods during the n revo utiot oft the wheels.n The cross-head 324. Compound para le ruler, composed on thel piston-rodn being attac ed to t e two l l of two simple rulers, A, A, connec ed by connec ing-rods, e piston-rodh is causedh o t t two crossed arms pi\·oted toge her a the mo e int a r ght ithe. t middle of heir leng , each p oted att one v i l n t th iv 329. A p ston-rod guide. T e p s on-rod, end to one of the rulers, and connec ed s i h i t t A, i connected wi h a wr s attached o a the o her one by a s ot a d t i t t w th sliding-pin, coo--wheel B turns on a crank-pin i t l n whic c, as s own a 13. In this he e ds as Ye � as h h t t n � l ca�ried b; a_'plate, w ic is fast on h; the edges ar_ie kep paralle . The pnnc1ple h h aroundt a t l ! shaft. The whee , B, revolves of co s ruct1on of the several ru_ ers repre- ! l D, of n � l stationary interna ly toothed gear, siented 1s take adva tage _iof the forma-1 l _i � n 11� double the diameter of B, and so motion is t1on of some par s of maclimer) • / t 0o-iven o the crank-pin'· and the piston-rod t is kept upr gh . 325. Parallel n1 er composed of t\YO sim­ i t ple rulers, A, B, connectedl by two pivoted 330. T e piston-rod is pro onged arid h l swinging arms, C, C. works i a guide, A, whic is in line w th h i t e centern of the cy inder. The lower part 326. A simyle means of guidi g or ob­ ofh t e connecting-rodl is forked to permit n taining a parallel motion of the pis on-rod the hupper par of he piston-rod o pass t of an eng e. The s ide, A, moves in and be ween. t t t in l t

- ! :i\,!ECHAXICAL l\fOVEl\IENTS. 83 ------·-·-·---·-·-···-···- ··- 331. An engine with crank motion like 336. An arrangement of 1 parallel motion that represented in 93 and 279 of this• for side lever marine engines. The paral- table, the crank-wrist journal working in a! lel rods connected with the side rods from slotted cross-head, A. This cross-head! the beams or side levers are also connected I works between the pillar guides, D, D, of with short radius anus on a rock-shaft the engine framing. working in fixed bearings.

332. A parallel motion used for the pis- 337. Parallel n1otion 1n which the radius ton-rod of side lever marine eno-ines.0 F C, l ' · 1 rod is connected ·with the lower end of a is the radius bar, and E the cross-head to . . . s11ort v1b rating roe,1 1 t 1e upper en d of ·w luch which the parallel bar, E, D, is attached. I is connected with the beam, and to tl1e cen­ l ter of which the piston-rod is connected. 333. A parallel n1otion used only in par- ticular cases. 338. Another modification, in which the

• 1 radius bar is placed above the beam. 334. Shows a parallel motion used 1n some of the old single-acting beam engines.

The piston-rod is formed with a straight 339. Parallel motion for direct action en- iI rack 0o-carincr 0 with a toothed segment on o-ines In this, the end of the bar, B, C, is 10 . the beam. The back of the rack works r connected with the piston-rod, and the end, ! · against a roller, A. ! B, slides in a fixed slot, D. The radius I bar , F , •A. ' is connected at F with a fixed I 335. A parallel motion commonly used I pivot, and at A, midway between the ends for stationary beam engines. j of B, C. I

-��======�======-=--, l\1ECI-iAXICAL MOVEMEXTS.

340. Another parallel motion. Beam, D, C, · gine. The cylinder has trunnions at its with joggling pillar-support, B, F, which ,·i- : upper end and swings like a pendulum. brates from the center, F. The piston-rodi• The crank-shaft is below, and the piston­ is connected at C. The radius-bar, E, A, I rod connected directly with crank. produces the parallel motion. I . i 346 . T abl e engme. The cy1- mcerl 1s· fixe d 341. "Grasshopper" beam engine. The 'ion a table-like base. The piston-rod has beam is atta<.:hed at one end to a rock- I! a cross-head workin in strafa�: ht slotted A, and the shaft ing-pillar, arranged as near ! guides fixed on top of cylinder, and is con- the cylinder as the crank will to work. 13 ! nected b., two side connectinrr-.-:, rods with is the radius-bar of the p;;i.rallel motion. ! two parallely cranks on shaft under the r 1asr. . . , table. 34::?. Old- h"i0nec l singl e - actmg beam ; pumping engine on the atmospheric prin- ] 347. Section of disk engine. Disk piston, ciple, with chain connection between piston- 'iseen edgewise, has a motion substantially rod and a segment at end of beam. The like a coin when it first falls after being cylinder is open at top. Very low pressurei, spun in the air. The cylinder-heads are steam is admitted below piston, and the ! cones. The piston-rod is made with a ball weight of pump-rod, etc., at the other encl ; to which the disk is attached, said ball of beam, helps to raise piston. Steam is ! ,Yorking in concentric seats in cylinder­ then condensed by injection, and a vacuum : heads, and the left-hand end is attached thus produced below piston, which is then I to the crank-arm or fly-wheel on end of forced down by atmospheric pressure there- , shaft at left. Steam is admitted alternately by drawing up pump-rod. on either side of piston. 343. Parallel motion for upright engine. 348. l\Iocle of obtaining two reciprocating A, A, are radius-rods connected at one end mo,·ements of a rod by one revolution of a with the framing and at the other with a shaft, patented in 1836 by B. F. Snyder, ,;ibrating piece on top of piston-rod. has been used for operating the needle of . . . . ar. sewing machine; by J. S. 1\fcCurdy, also 344. Osc1llatmg. engme.. The. cvhnder• , 1or 1 • • a o c,rinnrr rranrr0 f saws. 'fl1e d'1sk-, .�," has trunnions at the middle of its length · on tl 1e centra::, 1 i:,rotatmcr • s1 1a f t l1as t wo sI ots, working in fixed bearings, and the piston- . � . a. a crossmn- ting the same tend�ncy of rotating bodies. nectecl with an 1ntermed1ate bar, by wluch 1 This consists of three rings, A, A•, A 2, n:ieans the ends of the ruler, as well as the 1 place� one within the other and connected sides, are kept parallel. 1 by pivots at right angles to each other. 350. Traverse or to-and-fro motion. The 1 '.fhe smallest ring, A2, contains the bear­ pin in tl�e upper slot being stationary, and I mgs f�r the a?'is of_ a hea,:Y b�l, B.i. Tl�e the one ,n the lower slot 1nade to n1O,·e in l ball being set 1n rapid rotation, its axis will the direction of the horizontal dotted line ! continue in the same direction, no matter the lever will by its connection with the ba;i' how the position of the rings may be give to the latter a traversing motion in its ! �lte�ed ; �nd the ri!1g, A2, which supports rruides, a, a. : 1t will resist a considerable pressure tend­ i:, 351. Stamp. Vertical percussive falls de- t ing to displace it. rived from horizontal rotating shaft. The n1uti ated toothed p_in_ion acts upon the rack � _ i 357. What is called the g}TOSCOp'.e gover­ to raise the rod until its teeth leave the racki; nor, for steam engines, etc., patented by and allow the rod to fall. ,rss_8. i Alban Anderson in _A i_s a hea:y 352. Another arrangement of the Chinese . wheel: the axle, B, B , ot which 1s ma�e 1n illustrated by 129 of this table. ' two_ip_1eces connected tog�ther by a univer- windlass 0 _i 353. A modification of the tilt or trip sal J m!.i. The wheel, A, 1s onione p�ec';, B, l a _ hammer, illustrated by 72- J n this the . a?c pmi?n, I, on the othe� p1ec�, B • �he P!ece, -!J,. is Jt� b)" a hammer heh·e is a lever of the first order. : C?nnected at m:ddle 1 lunge JOint w_i th tl1e rev lv1i:ig �r ame H, In 74 it is a lever of the third order. . _ <;> ot . that ? the ; A . ! so variations 1n the mclmahon 3 4. mod1ficat1011of the crank and slot- wheel, A, will cause the outer end of the te� cross-head, 93. Tl_1 e cr<;>ss-head con- 1 ' piece, B, to rise and fall. The frame, H' is ta1�s an endless gro_ove_ 1n which the crank- ; driven by bevel gearing from the engine, wns! works, an� which 1s formed to produ_ce , and by that means the pinion, I, is carried a un1f< rm ve�oc1ty of movement of the wnst : round the stationary toothed circle, G, and or rec1procat_:i 1ng-rod. ! the wheel, A, is thus made to receive a rapid 355. The gyroscope or rotascope, an in- i rotary motion on its axis. \Vhen the frame, strument illustrating the tendency of rotat- ' H, and wheel, A, are in motion, the ten­ ing bodies to preserve their plane of rota- : dency of the wheel, A, is to assume a verti­ tion. The spindle of the metallic disk, C, : cal position, but this tendency is opposed is fittc:I to turn easily in bearings in the '. by a spring, L. The greater the velocity ria!?;. A. If the disk is set in rapid rotaryi, of the governor, the stronger is the tendency motion on its axis. and the pintle, F, at onei' above mentioned, and the more it o,;ercomes side of the ring, A. is placed on the bearing . the force of the spring, and ,,1 ice -;J ersa. The in the top ofthe pillar, G. the disk and ringi. piece, B, is connected with the valve-rod seem indifferent to gravity, and instead_ of : by rods, C, D, an� the spring, L, is con­ dr?pping begin to revolve about the vertical ' nected with the said rod by levers, N, and axis. I rod, P. i MECHANICAL l\t! OVE'.\1ENTS.

3;8. Traverse of carriage, made variable 362. Alternating traverse of upper shaft by fusee according to the variation in di­ and its drun1, produced by pin on the end ameter where the band acts. of the shaft working in oblique groove in the lower cylinder.

359. Primitive drilling apparatus. Being once set in 1notion, it is kept going by hand, 363. See-saw, one of the simplest illus­ by alternately pressing down and relieving trations of a limited oscillating or alternate the transverse bar to which the bands are circular motion. attached, causing the bands to wind upon the spindle alternately in opposite direc­ tions, while the heavy disk or fly-wheel 364. Intermittent rotary rnotion from con­ gives a steady mon1entum to the drill-spin­ tinuous rotary motion about an axis at right dle in its rot.rrv, n1otion. angles. Small wheel on left is driver ; and the friction rollers on its radial studs work against the faces of oblique grooves or pro- ljecti ns across he larger wheel, 360_ Continuous rotary motion from os� � � face of the n to and impart otron thereto. cillating. The beam being made vibrate, 1 the drum to which the cord is attached, working loose on fly-wheel shaft, gives mo- , tion to said shaft through the pawl and l 365. Cylindrical rod arranged between ratchet-wheel, the pawl being attached to ! two rollers, the axes of which are oblique drum and the• ratchet-wheel fast on shaft. to each other. The rotation of the rollers produces both a longitudinal and a rotary motion of the rod.

36r. Another simple form of clutch for pulleys, consisting of a pin on the lower 366. Drilling machir..e. By the large shaft and a pin on side of pulley. The pul- bevel-gear rotary motion is given to ver­ ley is moved lengthwise of the shaft by i tical drill-shaft, which slides through small means of a lever or other means to bring i bevel-gear but is made to turn with it by its pin into or out of contact with the pin on [ a feather and groove, and is depressed by shaft. ! treadle connecte� with upper lever.

=-=- r::::======-:-:--:..:--=--=--=-=- =-- =-=-=-=- =-= -==-- l\lECHANICAL NI OVE:.\1ENTS. ------·------367. A parallel ruler with which lines rotary motion to any piece of mechat!ism. may be drawn at required d stances apart The two hor zontal bevel-gears are arranged without sett ng out. �ower edgei of upper in a hoop-shapedi frame, wh ch re\·ofres blade has a graduatedi l\"Ory scale,_ 011 which freely on the middle of the horizontali shaft, the incidence of th� outer edge of the brass 011 which there are two vertical bevel-gears arc indicates the ,nclth between blades. ! gear ng to the horizontal ones, one fast and ; the otheri loose on the shaft. Suppose the 368. Describin� spir�l line on a cylinder. ! hoop to be held stationary, motion given to The spur-&"ear wluch dnv�s the bevel-�ears, '.either ,·ertical bevel-gear will be imparted and thus g1_ves rotary motion to the cylinder, i through the horizontal rrears to the �ther also gears 1nto the to?thed r:ick, and there- 1 vertical one ; but if the hoop be permitted by causes the marking J?Omt to traverse i it will revolve with the vert cal gear put in fromend to encl of the cyl nder. , motion, and the amount ofi power required i . . . 1, to hold it �tationary. w ll correspond w th 369 Cycloidal surfaces, causing pendulum i i . . that transmitted fron1 the first. rrear, and a to moY� in cycloidal curve, re�dering oscil- ( band attached to its periphery ,�ll indicate lations isochronous or equal-timed. j that power by the weio-ht0 required to keep 1 it st ll. 37?• i\lotion or polishing �1irrors, i _ f the 1 rubbmo- of which should be vaned as much ; as pract cable. The handle turns the crank '. .3�3- Robert's contrivan_ce for proving tl!at to whichi the lono- bar and attached ratchet- 1 fn ct1011 of a ,Yheel carnage does not in­ wheel are connetted. The mirror is secur- : crease "·ith velocity, but only with load. ed rigidly to the ratchet-wheel. The long • Loaded wagon is supported o_n s�irl�ce of bar, which is guided by pins in the loweri large "·heel, �nd c?nnect�d with mcl1cator ra l has both a loncritudinal and an oscillat- !: constructed with spiral spnng, to show force inrr'i mo,·ement and the ratchet-wheel is · required to keep carriage stationary "·hen ca�sed to rotdte intermittently by a click \ large ,�·heel s p1;1t n mo� on. It was found i i i operated by an e:centric on the crank-shaft, i t\1a� cl1�erence _m .\·eloc1ty prod�1�ecl no \'�­ and hence the mirror has a compound moYe- ; nation 1n the rnchcator, but difference 1n ment. : we ght immediately did so. ' i 371. od ficatio f _mangle-wheel mo- . �I i � o : 374. Rotary motion of shaft from trc:,dc t1on. 1 he large \\ he�l is. toothed on_ bot_ih . hv means band runnin� lrom faces, and an a) ternating orcular mot1�n is of an endless . _i a,roller on the treadle to an eccentric ·on the produced hy the uniform re,·olut�on ot the shaft. pin on, which passes from one side ?f the wheeli to the other through an openmg on .i . the left of the fi re. ( 37;. Pair of ed�e runners or c hascrs for gu , crushincr or ITT nding. The axles are con- 372. vVhite's dy?amometer, f?r determ!n- I nected � th �erti cal ·shaft, and the wheels or . ing the amount ot power required to give I chasers runi in ani annular pan or trough. i ! -. ··-····--- I

iVI ECHANICAL l\'IOVE'.IIE:'\TS. 93 . .--. --· ------•• ·••··------376. Tread-wheel horse-power turned by to recei\·e two wedges for clamping the the weight of an animal attempting to walk piece or pieces of wood to be planed. up one side of its interiori; has been used for drh·inob- the paddle-wheels of fe rry-boats• 382. Adjustable stand for mirrors, etc., and other purposes by horses. The turn- by which a glass or other article can be spit dog used also to be employed in such a raised or lowered, turned to the right or left, wheel in ancient times for turning meat and varied in its inclination. The stem is while roasting on a spit. fittedinto a socket of pillar, and secured by a set screw, and the glass is hinged to the stem, and a set screw is applied to the hinge 377. The tread-mill employed in jails in to tighten it. The same thing is used for some countries for exercising criminals con­ photographic camera-stands. demned to labor, and employed in grinding grain, etc. ; turns by weight of persons 383. Represents the principal elements of stepping on tread-boards on periphery. machinery for dressing cloth and warps, This is supposed to be a Chinese invention, consisting of two rollers, from one to the and it is still used in China for raising water other of which the , varn or cloth is wound, for irrigation. and an interposed cylinder having its peri­ phery either smooth-surfaced or armed with 378. Saw for cutting trees by motion of brushes, teasels, or other contrivances, ac­ pendulum, is represented as cutting a lying cording to the nature of the work to be tree. done. These elements are used in machines for sizing warps, gig-mills for dressing woolen goods, and in most machines for 379 and 3S0. Portable cramp drills. I11 finishing woven fabrics. 379 the feed-screw is opposite the drill, and in 380 the drill spindle passes through the 384. Helicograph, or instrument for de­ center of the feed-screw. scribing helices. The small wheel, by re­ \·olvirig about the fixed central point, de­ 3S r. Bowery's joiner's clamp, plan and scribes a volute or spiral by movir.g along transverse section. Oblong bed has, at one the screw-threaded axle either way, and end, two wedge-formed cheeks, adjacent transmits the same to drawing paper on sides of which lie at an angle to each other, which transfer-paper is laid with colored and are dovetailed inward from upper edge side downward. ···------'

MECHANICAL MOYEMENTS. 95 --·· ------'------38�- Contrivance emp�oy�d in Russia fori· crossed. The pulleys have attached to shutung doors. One pm . s fitted to and them pawls which encracre with two ratchet­ turns in socket attached to1 door, and the wheels fast on the flv1=wheel shait. One other is similarly attached to frame. In open- · pawl acts on its ratchet-wheel "·hen the ing the doo�, pins a�e _brought together, and piece, A, turns one way, and the other whe:1 \\i weigl�t is ra1se_d._ e1ght closes door by de- , the said piece turns the other way, and thus of the pressing the JOJnt toggle toward ai. a continuous rotarv., motion of the shaft is straight line, and so widening the space obtained. · between the pins.

. . . J 391. Reciprocating into rotary motion. F k 1ng library ladder. 386. � � It 1s showni; The weighted racks, A, A1, are piYoted to open, a• ti}' open, a � clo�ccl ; th _rounds -� � � I th_e end of a piston-rod, and pins at the end are pn oted to the s1de-p eces, "l11ch are , of the said racks work guide-crrooves, fi 1 in fixed tted together to form _a roun_d �ole when ; b, b, in such manner that one r�ck ope­ closed, the rounds shutting up inside. ' rates upon the cog-wheel in ascending and 1 the other in descending, and so continuous 387. Self-adjusting step-ladder for wharfs : rotary motion is produced. The elbow lever d, at which there are rise and fall of tide. The , C, and spring, are for carrving the pin ot1 steps are pivoted at one edge into woodeni· the right-hand rack over thc'upper angle in bars forming string-pieces, and their other , its guide-groove, b. • edge is supported by rods suspended from bars forming hand-rails. The steps remain . horizontal whatever position the ladder as- 39:2. Gig-saw, the lower end connected sumes. . f with a crank which works it, an� the upp�r , end connected with a spring which keeps 1t . strained without a gate. 388. Feed-motion of \V oostroke, and the bar, B, I - ste: entenng, ot course, 111 eft t_he un the · drops of its own gravity. opposite port.

,._ . . 1 4or. E P. Brownelrs patent ->96· G. P. Reen patent anchor nd �e, er crank-mo­ _ .n � tion to obviate dead-centers. The pressure escapem�nt for '� 5a tc_h es.n · The le er , is so i on the treadle causes the slotted slide, A, to comb1nat1on ". 1th_ chronom_eter I a.pphed i move forward with the ,Yrist until the latter em111nt that tl�e w�1ole impulse escap � . g:1venn; has passed the center, when the spring , balance in one direction 1s trans tte , B _ �1 cli fo rces the slide ao-ainst the stops until it is h f coun­ used on several sewino- machines for dr iv- ! teract1ng the effect of any sudden Jar upon ino- the shuttle. Sam� motion applied ton; a watch or ti1ne-piece. The jar which would th�ee-revol"•' >n cylinder printing-presses. accelerate motion of one wheel wot• ld r�- tard the motion of other. Anchor, A, 1s secured to lever, B, having an interior and 398. Continuous circular motion into in- exterior toothed seg ment at its encl, each termittent circular-the cam, C, being then1: one of which gears with the pinion of bal- driver. ! ance-wheels.