43 



wlicii to cjilciiUitc till' iivi-iafic (Ifiisilv nf tlic (.■iitirc piece llie <leii.sit.v of eaeli smaller |iR'ce is 



iimltiplied l).v the mean di.staiiceor this smaller piece Inim the center, ami the sii f the pHMluets 



divided l)y tlie sum of the distances. For it would not he sufficieut to add the (hMisities of tlio 

 several pieces, and divide this sum by the number of ]»ieces. in which ease the central (prismatic) 

 pieces would inordiinitely iiitiuence the result. 



For instiuice, a giveu pri.sm gave for the four sections from periithery to center the following 

 values : 



The density from simple adtlition woidd be -Lf^ = .4.5, while the method as used will give 



13.iJl — 51 

 3 2 — •'"^• 



Each iJiece is marked, first by the nninber of the tree, in Arabic; sec(md, by the nnnd)er of 



the disk, in Eoman numbers; ami if sjilit into small jiieces, each smaller piece by a letter of the 



alphabet, tlie piece at the periphery in all cases bearing the letter «. Besides the uuniber and 



letters mentioned, each ])ieee bears either the letter X. or S., to indicate its orientation on the north or 



south side of the tree. To illustrate: "> — vir X. a., means that the piect; bearing the label belongs 



to tree 5 and disk vii, comes ftom the north side of the tree, and is the peripheral part of this di.sk 



piece. From the collector's notes the exact position of this jiiece in the tre<- can readily be a.scer- 



tained. 



The entire prisms sent by freight are left in the original form, unle-ss used for .sjiecial imr- 



poses, and are stored in a dry room for future use. 



"WEIGHING AND JTKASURING. 



The weighing is done on an ajiothecary's balance, readily sensitive to 0.1 gram with a load of 

 more than 200 grams. Dealing with pieces of 200 to 1,000 grains in weight, the accuracy of weigh- 

 ing is always within 1 gram. 



The measuring is done by immersion in an instrument illustrated in the following design. 

 V is a vessel of iron, <Sf represents one of two iron standards attached to the vessel and ])i()- 

 jecting above its top; B is a metal bar fastened to the cii]) -1, which serves as guard to the cup 

 and i)revents it going down further at one time than another by coming to rest on the standards S. 

 The cu]) A dips down one-.sixteenth to onti-eighth of an inch below the edge of tlie knee-like sjiout. 

 In working, the cup is lifted out by tlie handle which the bar />' forms, water is |)Oured into the 

 vessel until it overflows through the spout, tlieu the cup is set down, replacing tlie nioliile and 

 fickle natural water level by a constant artiticial one. Now the instrument is set, the )ian /' 

 is])laced under the spout, the cup is lifted out and held over the vessel, so that the drijijiiiigs fall 

 ba<-k into the latter, the i>iece of wood to be measured is put into the vessel and the ciiji re])laced, 

 and pressed down until the bar /.* i-ests on the standards iV. This is done gently to pre\-eiit the 

 water from ri.sing aboxc tlie rim of the vessel. This latter |irccaution is suj)erfliious where tlic cup 

 fits closely, as it does in one of the instruments thus far u.sed. The jian with water is then weighed, 

 the pan itself being tared by a bag of shot. The water is poured out, the pan wiped dry, and 

 the process begins anew. To work well it takes two persons, one to weigh and record. The 

 water pan is a .seamless tin pan, holding about l,o(>0 cc. of water and weighing only 1)4 grams. 

 The temperature as well as density of the water are ascertained, the latter, of cour.se, omitted 

 when distilled watei- is used. To maintain the water at the .same temperature it reiiuires frei|uent 

 changing. 



In constructing the apparatu.s, care should be had that t Ik- point X is higher than lliejioint 

 M; if this is not the <Mse, some air. slightly comiiressed by a closely fitting cui>, .suddenly finds an 



