108 KEPOKT— 1881. 



several trials I found that the pressure of a few pounds with one hand only 

 was quite sufficient to produce an effect. 



It must be remembered that this is not a case of a small pressure 

 delivered on the bare earth at say 7 feet distance, but it is the difference 

 of effect produced by this pressure at 7 feet and 8 feet ; for of course the 

 change only consisted in the change of distribution in the weight of a 

 small portion of my body. 



We have, however, since shown that even this degree of sensitiveness 

 may be exceeded. 



We had thought all along that it would ultimately be necessary to 

 take our observations from outside the room, but this observation im- 

 pressed it on us more than ever ; for it would be impossible for an observer 

 always to stand in exactly the same position for taking readings, and my 

 brother and I could not take a set of readings together on account of the 

 difference between our weights. 



In making preliminary arrangements for reading from outside the 

 room we found the most convenient way of bringing the reflected image 

 into the field of view of the telescope was by shifting a weight about the 

 room. My brother stood in the room and changed his position until the 

 image was in the field of view, and afterwards placed a heavy weight 

 where he had been standing ; after he had left the room the image was in 

 the field of view. 



On the S.W. wall of the room there is a trap-door or window which 

 opens into another room, and we determined to read from this. 



In order to read with a telescope the light has to undergo two reflec- 

 tions and twelve refractions, besides those in the telescope ; it has also to 

 pass twice through layers of water and of the fluid mixture. In con- 

 sequence of the loss of light we found it impossible to read the image of 

 an illuminated scale, and we had to make the scale self-luminous. 



On the pavement to the S. of the instrument is placed a flat board on 

 to which are fixed a pair of rails ; a carriage with three legs slides on 

 these rails, and can be driven to and fro by a screw of ten threads to the 

 inch. Backlash in the nut which drives the carriage is avoided by means 

 of a spiral spring. A small gas-flame is attached to the carriage ; in front 

 of it is a piece of red glass, the vertical edge of which is very distinctly 

 visible in the telescope after reflection from the mirror. The red glass was 

 introduced to avoid prismatic effects, which had been troublesome before. 

 The edge of the glass was found to be a more convenient object than a 

 line which had been engraved on the glass as a fiducial mark. 



The gas-flame is caused to traverse by pulleys driven by cords. The 

 cords come to the observing window, and can be worked from there. A 

 second telescope is erected at the window, for reading certain scales 

 attached to the traversing gear of the carriage, and we find that we can 

 read the position of the gas-flame to within a tenth of an inch, or even 

 less, with certainty. 



From the gas the ray of light enters the tub and mirror-box, is 

 reflected by the mirror, and emerges by the same route ; it then meets a 

 looking-glass which reflects it nearly at right angles and a little upwards, 

 and finally enters the object-glass of the reading telescope, fixed to the 

 sill of the observing window. 



When the carriage is at the right part of the scale the edge of the 

 red glass coincides with the cross wire of the reading telescope, and the 

 reading is taken by means of the scale telescope. 



