80 



or ncarlv so, for large a.s well as for small displacements, and there is no sensilile 

 rotation of the steady lino, during displacement, ahont tlie axis of support. 



The moment of frictional resistance, due to the pivots and pointer, must Ijc 

 very small relatively to the moment of the effective inertia. 



The dis])lacement of the multiplying pointer, in the jjlane of the surface on 

 which it writes, mu.st lie a constant multiph' of the displacement of the ground 

 with respect to the steady line. Tlie jiointer must, however, have freedom to 

 move, through a small distance at least, at right angles to tlie .surface on which 

 it writes. To give it this freedom the writer has, after several trials, found no 

 plan so satisfactory as the use of a transverse joint, examples of which will be 

 found in all his seismograplis. 



The record-receiving j>late or drum nuist have its axis of rotation deKnitely 

 attached to the earth, or, preferably, to a i)icce which is rigidly connected with 

 the axis of snp];ort of the seismometer who.«e disjdacement it is to record ; and 

 the n)otion of the record-receiver must be .steady and continu.ms during an carth- 

 ([uake. These conditions are fulfilled in the writer's ajiparalus by the indejen- 

 dcnt supporting frame which is used to carry the gla.ss ])late, without shake; the 

 rolling-contact gearing, in place of toothed wheels, which, unices made with 

 extreme accuracy, would give un.'tcadiness to the motion ; and the governor, 

 who.se action is not sensibly s] asniodic, and whicii is designed with a special view 

 to its being undisturbed by the motion of tlie ground. To fix the snioked-gla.«s 

 ])late directly to a slow-running arbor of the clock, ;hough apparently a simpler 

 method of suj)]K)rt, gives moic chance of irregular motion, and a considerable 

 likelihood that the jjlate may shake. 



A glass sm-facc is preferable to j-a] er for the receptidU of kcokIs, on account 

 of its smoothne.'s; and a plate is in many ways more ronxenieiu than a drum.* 



* To obtain permnneiit copii's of a record traced on a smohedglass plate, the plate is fii'st to 

 1 <; coalod w itli |jliuloj,'n>jilicr's vaniiFli, 1 y pourinn llip liquid on it, and gontly sloping it to allow 

 tlie variiisli to spread uniformly. After it is dry, the "blue process'' of talking pliolograjihs uiuy 

 be applied as follows. - 



dissolve one part of Ainmonio-Cilratp of Iron in eight parts of water, and niahe a separate 

 solution containing one j.art of Ferric\anide of rolassinni aUo dissolved in eight parts of water. 

 Immediately before taldng the iihotogniphs nii.\ four jiarts of llie former solution with three 

 parts of the latter. Spread the mixture evenly over a sheet of pajier (either with a glass rod or 

 with a flat brush) and hang tlie paper up to dry in a dark room. AVIiendry, e.\pose it under the 

 glass jilate, with the varnished side next to the jiajier, care being taben lo bring the paper and the 

 plale into immediate contact. This is most easily done liy laying tlie sensitized papiT on a thick 

 cushion consisting of several layers of cloth, and then pressing the gla.ss plate firmly against it 

 by means of a second sheet of plaleglass which may be held down by clamps. In sunshine 

 the exposure should last from eight to twenty minutes. 



After exposure, the paper is to be dipped into a lank of water an<l allowed to soak there for 

 about ten minutes. It must be thoroughly washed, so that every piirt of the unaffected chemicals 

 on the paper may bo removed. It may be then dried in the open air. 



The solution of Ammonio-Citrate of Iron shcjuld be kept in a dark place; and it is best to 

 prepare it not long before use. The sensitized paiier may be prejiared in large (juantities and 

 laid by in a dark drawer : but if it is kept long the jihotographs are not so clear as those oljtaincd 

 by the use of freshly prepared pajier. 



