an autobiography-第83部分

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ll　cases　proved　most　handy　and　satisfactory。

This　method　consisted　of　tinning　three；　or；　if　need　be；　more　parts　of　the　work；　and　laying　them　down　on　a　tinned　face…plate　or　chuck；　which　had　been　heated　so　as　just　to　cause　the　solder　to　flow。　　As　soon　as　the　solder　is　cooled　and　set；　the　chuck　with　its　attached　work　may　then　be　put　in　the　lathe；　and　the　work　proceeded　with　until　it　is　completed。　　By　again　heating　the　chuck；　by　laying　upon　it　a　piece　of　red…hot　iron；　the　work；　however　delicate；　can　be　simply　lifted　off；　and　will　be　found　perfectly　free　from　all　distortion。

I　have　been　the　more　particular　in　naming　the　use　of　three　points　of　attachment　to　the　chuck　or　face…plate；　as　that　number　is　naturally　free　from　any　risk　of　distortion。　　I　have　on　so　many　occasions　found　the　great　value　of　this　simple　yet　most　secure　mode　of　fixing　delicate　work　in　the　lathe；　that　I　feel　sure　that　any　one　able　to　appreciate　its　practical　value　will　be　highly　pleased　with　the　results　of　its　employment。

The　same　means　can；　in　many　cases；　be　employed　in　fixing　delicate　work　in　the　planing…machine。　　All　that　is　requisite　is　to　have　a　clean…planed　wrought…iron　or　brass　fixing…plate；　to　which　the　work　in　hand　can　be　attached　at　a　few　suitable　parts　with　soft　solder；　as　in　the　case　of　the　turning　lathe　above　described。


1828。　A　Method　of　casting　Specula　for　Reflecting　Telescopes；　so　as　to　　　　　　　ensure　perfect　Freeness　from　Defects；　at　the　same　time　enhancing　　　　　　　the　Brilliancy　of　the　Alloy。

My　father　possessed　a　very　excellent　achromatic　spy…glass　of　2　inches　diameter。　　The　object…glass　was　made　by　the　celebrated　Ramsden。　When　I　was　about　fifteen　I　used　it　to　gaze　at　the　moon；　planets；　and　sun…spots。　　Although　this　instrument　revealed　to　me　the　general　characteristic　details　of　these　grand　objects；　my　father　gave　me　a　wonderful　account　of　what　he　had　seen　of　the　moon's　surface　by　means　of　a　powerful　reflecting　telescope　of　12　inches　diameter；　made　by　Short　that　justly　celebrated　pioneer　of　telescope　making。　　It　had　been　erected　in　a　temporary　observatory　on　the　Calton　Hill；　Edinburgh。　　These　descriptions　of　my　father's　so　fired　me　with　the　desire　to　obtain　a　sight　of　the　glorious　objects　in　the　heavens　through　a　more　powerful　instrument　than　the　spy…glass；　that　I　determined　to　try　and　make　a　reflecting　telescope　which　I　hoped　might　in　some　degree　satisfy　my　ardent　desires。

I　accordingly　searched　for　the　requisite　practical　instruction　in　the　pages　of　the　Encyclopedia　Britannica；　and　in　other　books　that　professed　to　give　the　necessary　technical　information　on　the　subject。　　I　found；　however；　that　the　information　given　in　booksat　least　in　the　books　to　which　I　had　access　was　meagre　and　unsatisfactory。　　Nevertheless　I　set　to　work　with　all　earnestness；　and　began　by　compounding　the　requisite　alloy　for　casting　a　speculum　of　8　inches　diameter。　　This　alloy　consisted　of　32　parts　of　copper；　15　parts　of　grain　tin；　and　1　part　of　white　arsenic。　These　ingredients；　when　melted　together；　yielded　a　compound　metal　which　possessed　a　high　degree　of　brilliancy。　　Having　made　a　wooden　pattern　for　my　intended　8…inch　diameter　speculum；　and　moulded　it　in　sand；　I　cast　this　my　first　reflecting　telescope　speculum　according　to　the　best　book　instructions。　　I　allowed　my　casting　to　cool　in　the　mould　in　the　slowest　possible　manner；　for　such　is　the　excessive　brittleness　of　this　alloy　（though　composed　of　two　of　the　toughest　of　metals）　that　in　any　sudden　change　of　temperature；　or　want　of　due　delicacy　in　handling　it；　it　is　very　apt　to　give　way；　and　a　fracture　more　or　less　serious　is　sure　to　result。　　Even　glass；　brittle　though　it　be；　is　strong　in　comparison　with　speculum　metal　of　the　above　proportions；　though；　as　I　have　said；　it　yields　the　most　brilliant　composition。

Notwithstanding　the　observance　of　all　due　care　in　respect　of　the　annealing　of　the　casting　by　slow　cooling；　and　the　utmost　care　and　delicate　handling　of　it　in　the　process　of　grinding　the　surface　into　the　requisite　curve　and　smoothness　suitable　to　receive　the　final　polish；　I　was　on　more　than　one　occasion　inexpressibly　mortified　by　the　sudden　disruption　and　breaking　up　of　my　speculum。　　Thus　many　hours　of　anxious　care　and　labour　proved　of　no　avail。　　I　had　to　begin　again　and　proceed　da　capo。　　I　observed；　however；　that　the　surplus　alloy　that　was　left　in　the　crucible；　after　I　had　cast　my　speculum；　when　again　melted　and　poured　out　into　a　metal　ingot　mould；　yielded　a　cake　that；　brittle　though　it　might　be；　was　yet　strong　in　comparison　with　that　of　the　speculum　cast　in　the　sand　mould；　and　that　it　was　also；　judging　from　the　fragments　chipped　from　it；　possessed　of　even　a　higher　degree　of　brilliancy。

The　happy　thought　occurred　to　me　of　substituting　an　open　metal　mould　for　the　closed　sand　one。　　I　soon　had　the　metal　mould　ready　for　casting。　It　consisted　of　a　base　plate　of　cast　iron；　on　the　surface　of　which　I　placed　a　ring　or　hoop　of　iron　turned　to　fully　the　diameter　of　the　intended　speculum；　so　as　to　anticipate　the　contraction　of　the　alloy。　The　result　of　the　very　first　trial　of　this　simple　metal　mould　was　most　satisfactory。　　It　yielded　me　a　very　perfect　casting：　and　it　passed　successively　through　the　ordeal　of　the　first　rough　grinding；　and　eventually　through　the　processes　of　polishing；　until　in　the　end　it　exhibited　a　brilliancy　that　far　exceeded　that　of　the　sand　mould　castings。

The　only　remaining　difficulty　that　I　had　to　surmount　was　the　risk　of　defects　in　the　surface　of　the　speculum。　　These　sometimes　result　from　the　first　splash　　of　the　melted　metal　as　it　is　poured　into　the　ring　mould。　The　globules　sometimes　got　oxidised　before　they　became　incorporated　with　the　main　body　of　the　inflowing　molten　alloy：　and　dingy　spots　in　the　otherwise　brilliant　alloy　were　thus　produced。　　I　soon　mastered　this；　the　only　remaining　source　of　defect；　by　a　very　simple　arrangement。　In　place　of　pouring　the　melted　alloy　direct　into　the　ring　mould；　I　attached　to　the　side　of　it　what　I　termed　a　〃pouring　pocket；〃　which　communicated　with　an　opening　at　the　lower　edge　of　the　ring；　and　by　a　self…acting　arrangement　by　which　the　mould　plate　was　slightly　tilted　up；　the　influx　of　the　molten　alloy　advanced　in　one　unbroken　tide。　　As　soon　as　the　entire　surface　of　the　mould　plate　was　covered　by　the　alloy；　its　weight　overcame　that　of　my　up…tilting　counterpoise；　and　allowed　the　entire　apparatus　to　resume　its　exact　level。　　The　resulting　speculum　was；　by　these　simple　arrangements；　absolutely　perfect　in　soundness。　　It　was　a　perfect　casting；　in　all　respects　worthy　of　the　care　and　labour　which　I　invested　in　its　future　grinding　and　polishing；　and　enabled　it　to　perform　its　glorious　duties　as　the　grand　essential　part　of　a　noble　reflecting　telescope！

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A。　Chill　plate　of　cast　iron　turned　to　the　curve　of　the　speculum　B。　Turned　hoop　of　wrought　iron　with　opening　at　O。　C。　Pouring　pocket。　D。　Counterpoise；　By　which　the　chill　plate　is　tilted　up　The　largest　figure　in　the　engraving　is　the　annealing　tub　of　cast　iron　filled　with　sawdust；　where　the　speculum　is　placed　to　cool　as　slowly　as　possible。

The　rationale　of　the　strength　of　specula　cast　in　this　metal　mould　system；　as　compared　with　the　treacherous　brittleness　of　those　cast　in　sand　moulds；　arises　simply　from　the　consolidation　of　the　molten　metal　pool　taking　place　first　at　the　lower　surface；　next　the　metal　base　of　the　mouldthe　yet　fluid　alloy　above　satisfying　the　contractile　requirements　of　that　immediately　beneath　it；　and　so　on　in　succession；　until　the　last　to　consolidate　is　the　top　or　upper　stratum。　Thus　all　risk　of　contractile　tension；　which　is　so　dangerously　eminent　and　inherent　in　the　case　of　sand…mould　castings；　made　of　so　exceedingly　brittle　an　alloy　as　that　of　speculum　metal；　is　entirely　avoided。　By　the　employment　of　these　simple　and　effective　improvements　in　the　art　of　casting　the　specula　for　reflecting　telescopes；　and　also　by　the　contrivance　and　employment　of　mechanical　means　for　grinding　and　polishing　them；　I　at　length　completed　my　first　8…inch　diameter　speculum；　and　mounted　it　according　to　the　Newtonian　plan。　　I　was　most　amply　rewarded　for　all　the　anxious　labour　I　had　gone　through　in　preparing　it；　by　the　glorious　views　it　yielded　me　of　the　wonderful　objects　in　the　heavens　at　night。　　My　enjoyment　was　in　no　small　degree　enhanced　by　the　pleasure　it　gave　to　my　father；　and　to　many　intimate　friends。　　Amongst　these　was　Sir　David　Brewster；　who　took　a　most　lively　and　special　interest　in　all　my　labours　on　this　subject。

In　later　years　I　resumed　my　telescope　making　enjoyments；　as　a　delightful　and　congenial　relaxation　from　the　ordinary　run　of　my　business　occupations。　　I　constructed　several　reflecting　telescopes；　of　sizes　from　10…inch　to　20…inch　diameter　specula。　　I　had　also　the　pleasure　of　assisting　other　astronomical　friends；　by　casting　and　grinding　specula　for　them。　　Among　these　I　may　mention　my　late　dear　friend　William　Lassell；　and　my　excellent　friend　Warren　de　la　Rue；　both　of　whom　have　indelibly　recorded　their　names　in　the　annals　of　astronomical　science。　　I　know　of　no　subject　connected　with　the　pursuit　of　science　which　so　abounds　with　exciting　and　delightful　interest　as　that　of　constructing　reflecting　telescopes。　　It　brings　into　play　every　principle　of　constructive　art；　with　the　inexpressibly　glorious　reward　of　a　mor