'">•> 



])ass; tliioii>;Ii the |ioiiit of inU'isectiou of a liurizunhil liiir tlinuifih b and a vorti- 

 cal lino throiiiili tlio ooiitro of »^ravitv of tlio wliolo Mispoiulod mas-^. Tho soicw 

 f i.s lixod t'l tiio r^tocl framo once for all, Imt the upju'i' scrow /) is adjiistaliit' ami 

 is jn'ovidoil willi a jani-niit, in nidcr that tlio axis uf >u]i] ml may lie I)riJiiu'ht inti> 

 ]iarallolisiu with tho axi.s of tho iii'li. Tlio V-slot and oonioal holos intu \viii<li 

 tho various points jiross aro out with a nioro ol'tnso ani-lo than tho points whioii 

 ontor then). Tho pointor is a litrht rood fiirnisliod at its writing' ond with a stool 

 jioint, onnsistin-r o( a picoo id' Ixnt watch-siniiiL;- tihd >liai|i and i;lass-hardcnod. 

 Its lonjTth is siioh as to irivo a rooord which is fotir times tiie actual displacomoiit 

 of tho ground. Tho pointer is jointed to tlie stool franio at d (tig. 7) and tlio 

 groator part of its weight is horno hv a spiral spring of fine wire, hanging from a 

 i>ent arn> faston'd to tho fiamo liy a iiiiuhiiig-screw at c, which allows tin- 

 toii.-ion of tho spring to ho adjusted l)v raising or lowering tlie end of the arm. 

 Tho spring supjiorts the j^ointer near the joint and somewhat helow tiie central 

 lino, an arrangcniont which permits tho pressure of the marking end on the glass 

 jilato to he kept .«en.sihly constant thn iigli(.iit a con.«ideral)lo range of up-and- 

 down movement of tho ]ilate. 



The glass ])late is cairied l>y a veitical >U-v\ spindle juvotted in a wooden 

 .stan<l between an agate cnp at the hottom and a steel screw at the top, as is shown 

 in tiguiv 5. To enahle the ])late to ho readily rciiiovcil and a fresii one ]int in, 

 it is attached to tho spindle thus: — a cinailar lira-s ]ilate /' is permanently fixed 

 to the spindle, and carries three inverted levelling-screws on whoso points (which 

 are covered with chamoi.s-skin 1 the glass plate 7 re.sts. Next comes a suit Icatiier 

 wa.-iher and above it a .stout bra^.s wa-slier h, which is pressed down on tho plate by 

 a wedged -.';hapc<l key i fitting in a slot in tho spindle. Tiie levelling-screws in/ 

 allow the plane of the |)]ate to bo oasilv aiijustod : a rough adjustiiienf is enough, 

 since the jointo<l pointers arc able to follow any up-and-down movements which 

 the sinface of the plate may make ;ls it revolves. 



Tho driving clock is an ordiiiarv train of wheels driven by a weight of ten 

 kili>s., wliicli is wound up once a <lay and hiia fall of aliout live metres. Tho speed 

 is regulated iiy a governor whose con.stniction will liee|e:ir IVom fig. (I. It eon- 

 .-ists of a vertical spindle carrying a pair of jointed rods, each provided with a 

 pair of l)i-.Lss balls, and eai-h carrying a fan wiiieh dips into a trough of oil when 

 the governor n-voives. The centrifugal tendency of the balls i.s partially resisted 

 liv a pair of springs which tie tliom to tlio .spindle. The govornoi' receives its 

 motion by rollini; contact lwtwei>n a ili-^- fixed on the voi tical spindle and another 

 di-c on ihc ht-t axle of the clock-train. 'I'lie clock drives the glass ]ilate by a 

 roller /.-, whos«' axle is made tjexibie at one point ( / ) in order thai it may ailapt 

 it.s<-lf (n the varying lieight of the surface. Tliis is done by cutting the axle at I 

 and connecting the ends by a single turn of steel wiii', which I'orms a convenient 

 .•■ubstitute (or a IIiMik<''s joint. The axle of the loller pas-ses through a guide m 

 H'm. 8), and clox' to the slot in wliieli it workn Lü a second slioi(i'r slot //. ]W 

 lifting the axle into this, A* i.s mi.M.-d out of contact with the plate, whose motion 



