k; 



wliicli would 1k' so (lesiiablo. It. inust ho icmeinlioveil, tluMi, tliat tlie results do 

 not possess tiuit value as ahsolute wave-leiigtlis which I liolicve would othciwise 

 Ixjlong ti) thciu. Their rchit'ii-c value is not altered, however, hy tliis faet and 

 f lio|>e to have fiuiiislied the data l)y means of which, at any iutiu'e time when 

 it lieeonies possilile. eneh slight eorreclions as may he neeessary may he easily 

 aii|ili('il. rt is lie]ie\(Ml that tin- \alui' nl" \\\r grating sjjaoe assnmi'il is rcr// 

 (•/«sc to the true value and lh;il the r<incctinns to In' applird hcrcaltci- will In- 

 very small. 



The following are (he facts in regard lo the grating space as far as at 

 present, known. 



The grating is markeil by the maker '" \11'M\ lines to one inch." I am 

 also informed hy JTr. f'hapman that it is atfem|ited to ki'Op the temiterature as 

 nearly constant as |io,ssiiil(; at C>^iP F. during tlie process of ruling', hut that as 

 till' rnlinii of a larger grating occupies sever.al days the temperature often lluctuatcs 

 two or threii degrees above and helow that paint. The tenjjierature to which 

 ihefie oliservation.s have all In-en reduced is IS'^, whicli is very nearly that at 

 wliirh the ruling was niadt^ This nundicr, 1Tl!!)<I to on<' inch, is equiv.ileid to 

 ()8().9ö7 to one millimetre and were the inch <if the machine knuwu to he correct 

 this munhiu- nuglit he jiccepted. I'nt Jfr. ('iiapman says that one English inch 

 is nndoiditeilly less than M'l'M'i of thes(i di\isions hy one or two divisions. 



in his ••Measurements of Gravity at fiutial Stations in America and 

 I'jurope" ■• j\lr. Peiroe has given the result of an (dahorate examinal ion of a centi- 

 metre ruled upon ]\Ir. Itutherfiird's m.iehine. The concluding sentence of this 

 discussion is as follows; — 



''This centimetre is c(|ual to GSOOi toetii <if RntherfonVs machine, and as 

 ''■ ^^ jciöiiiT *"" l'^"Jj;! we conclude that (JSOO teeth make a centimetre at ordinaiy 

 temperatures, say ahout 18'^." As this temperature agree« witli that at which 

 the grating was ruled and also that to which the ohsiavations have been rednoiMJ, 

 it is the most accurate determination at present available .and in all of the 

 following reductions tlu^ S]iaces have been assumed to be (iSD!) to one millimetic. 

 This number is not free from doubt, however, as is evident from the fact that 

 the tempera! luc at which this piu'tii-idar grating was ruletl may not b(^ exactly 

 identical with that of the above comparison and besides there is room for (had it as to 

 tlieacnuraoy of the decimetre of comparison and Mr. Peirce justly remarks that ''all 

 <'X!xet measures of length inad(^ now- must wait for their (hial correction until the 

 establishment of th(^ new metric |)rototy]ie"'. It is likely therefore that the 

 assnn)ption made is tla^ best possilile under tlxs eircuuistances. 



In ord<a- that it might be [lossible to correct these ri^snlts, if necessary, iiy 

 means id' the metliod of comparison of angular measures, I have endeavoied to 

 conni'i-t the series with .Mr. I'cii-cc"s angular measures for th(r comparison of a 

 wave-length with a metre, {''or this |iur]iosi' I havi^ referre.l Id the mdy p^bli^lled 



* Appendix No 15 — Report of 1876— Ü. S. Coast Survey.— IVinted at Washington— 1879. 



