On Gravimeters. 



519 



the design. I shall, therefore, describe the instrument as I see it, 

 with direct reference to what I conceive to be the intention.* 



The gravimeter stands, when set up for use, on a tripod ; but it 

 might equally be stood upon a table with a bay cut out of the side 

 two or three inches deep. In either case it would stand on three ad- 

 justing foot-screws carrying a thick brass foundation plate. Below this 

 plate projects what I will call the well and a cathetometer. Above it 

 stands the chamber, consisting of two brass plate sides and glass 

 plate front and back, besides various clamping and manipulating 

 appliances. On the top of the chamber are two cross levels, and from 

 the centre rises a hollow shaft. 



Measuring from the surface of the foundation plate, the depth of 

 the well is about 8^ inches (26 centims.), and the height of the chamber 

 inches (16 centims.). The shaft stands about 18 J inches (47 

 centims.) above the plate, or 13 inches above the roof of the chamber. 

 The latter is about 2 J inches wide and 2-f inches deep (say 6 centims. 

 each way) . This general description will suffice to give an idea of the 

 framework. More minute descriptions will follow as they become 

 necessary. 



Within the chamber are suspended by wires, one below the other, 

 two weights, which for distinction I will call the major weight and the 

 minor weight. The major weight consists of a brass cubical block, 

 from which rise two lateral rectangular pillars, crossed at top by a 

 somewhat slighter bar. The block is perforated vertically by a large 

 cylindrical hollow. This major weight is suspended by two parallel 

 wires which traverse the shaft ; being just visible at its lower end, 

 where they enter two small screws on the cross-bar. There are means 

 of adjusting both their length and distance apart, above and below. 



The principle here is that of the bifilar torsion balance : if the block 

 be turned through any angle less than 180°, its tendency to return is 

 (mainly) due to the weight of the mass suspended. This, however, is 

 not that of the major weight only ; for from the under side of the 

 cross-bar hangs a single wire, which, descending through the hollow r 

 in the block, is attached to and supports the minor weight ; hence the 

 mass suspended by the double wire is the sum of the masses of the 

 two weights. 



The minor weight consists of a light frame, which I will call the 

 head, and a long thin glass rod or plunger, which descends into the 

 well. 



Suppose the major weight turned through an angle 6. When the 

 disturbance has subsided and oscillations have been quelled, it will, of 

 course, be found that the minor weight has turned through a like 



* Since this report was written, two plates have been drawn to illustrate the 

 description which follows. Necessarily there is no direct reference to them in the 

 text. 



