132 LECTURES TO SCIENCE TE AGREES. 



the bubble upon the scale furnishes a correct indication of 

 the amount of action due to the increased or diminished 

 force of gravitation on the instrument all other conditions 

 being equal. Another cause of disturbance on the instru- 

 ment is due to the pumping action of the ship. A ship in 

 going over the ocean, as we all know some of us from 

 unpleasant experience has an upward and downward 

 motion ; and disagreeable as it is to us, it is still more 

 disagreeable to this instrument, unless it is so arranged as 

 to represent a good sailor. The mode in which this difficulty 

 has been dealt with consists in stopping up the upper end of 

 this tube, leaving only a very small cylindrical perforation 

 for the flow of the mercury in one direction or the other. 

 The diameter of this hole is only about one-tenth of a 

 millimetre, and in that opening we still find a pumping 

 action equal to about 10 millimetres on this scale that is 

 to say, when the ship goes up and down this air-bubble 

 moves 10 millimetres to and fro, and we have to get our 

 reading by taking the mean between those two positions. 

 If this motion was much greater than 10 or 15 millimetres, 

 it would be difficult or impossible to get accurate readings 

 while the ship was in motion, but it is not so difficult to get 

 the true reading with a variation of so small an amount. 



It was important to ascertain what the influence of 

 latitude was upon this instrument in order to get the true 

 comparison between the influence of latitude and the 

 influence of depth, and here I have met with a result which 

 requires further explanation. The influence of this instru- 

 ment moving over a varying depth of the sea has been well 

 ascertained in sending it several times across the Atlantic 

 ocean. I have also tried to get the influence of latitude 

 determined by sending it to Brighton, back to London, and 

 up to Scarborough ; and I have observed that while the 

 influence of the depth of the sea is rather in excess of 

 what our calculations had given, the influence of latitude is 

 less than calculation would imply ; and it remains to be 

 seen how these discrepancies can be explained. But so 

 much is certain, that according to all observers, the gravi- 

 tation on the sea- shore is influenced by a variety of causes, 

 such as the attraction of continents, the density of the mass 

 immediately below; and I have no doubt that further 

 observations will clear up this apparent discrepancy. I 



