THE BATHOMETER. 127 



hypothesis, and I doubt that Dr. Hann takes the attraction 

 too much in the sense of astronomical attraction rather than 

 in the sense of local attraction. Here I would refer to an 

 experiment which we all know and can easily make. Any 

 large substance falls towards the centre of the earth. If 

 we throw a weight against a wall it falls vertically down 

 to the earth, but if we throw a drop of water against the 

 wall it does not fall towards the earth it adheres to the 

 wall ; and if we throw a small particle of sand against the 

 wall it does not fall, it adheres to the wall. This is ex- 

 plained by the adhesion. Adhesion is only another form 

 of attraction. It is attraction at very small distances ; and 

 for the grain of dust hanging against the wall, the centre 

 of attraction of the whole earth may be thought of as 

 concentrated in the wall, and as falling within the sub- 

 stance of the wall, because this local attraction is so great 

 owing to its vicinity to the particle that it overcomes 

 actually the greater attraction of the total earth. And so 

 a small insect mounting up a wall would probably be in- 

 fluenced more by the attraction .of the wall, if it is a massive 

 one, than by the total gravitation of the earth ; and it 

 must always be borne in mind that the attraction of the 

 substance nearer at hand exercises an influence which is 

 greater than the mass at a distance, in the proportion of the 

 square of that distance, which is a very large proportion. 



The next problem was the construction of such an 

 instrument, and this presented considerable difficulties, 

 because we have to deal with exceedingly small variations 

 of total gravitation, and we have to deal with disturbing 

 influences which should be eliminated or allowed for. 



I will now describe the instrument. One is placed 

 before you of the size which I have constructed at present, 

 and an enlarged drawing is placed upon the wall. The 

 total gravitation of the earth is represented by a column 

 of mercury which rests upon a thin diaphragm of steel plate. 

 This diaphragm of steel is embossed in such a way. that its 

 centre can move within a small range freely up and down 

 under the influence of a column of mercury without en- 

 countering any frictional resistance. The mercury column 

 ends again in a cup \ and the whole system, the pipe and the 

 cup, are filled with mercury up to the line shown in red on 

 the diagram. The space above the mercury is filled up 



