THE BATHOMETER. 135 



can be increased, however much or however little, I think 

 it is an object well worthy of effort. 



If I may be allowed a few minutes more, I will refer to 

 a corresponding instrument for measuring horizontal 

 attractions. There is an imperfect diagram suspended 

 here and the instrument is down stairs. The object is to 

 measure local attraction in a horizontal direction, and it 

 serves to illustrate the principles upon which the batho- 

 meter is based. There are two bulbs connected longitu- 

 dinally by means of an iron tube, and the iron tube and 

 the bulbs up to their mid-centre are filled with mercury. 

 The mercury naturally will assume a horizontal position 

 in the two bulbs, except when influenced by a horizontal 

 force. Such a force would have a tendency to draw the 

 mercury in the tube towards it, and this attraction 

 exercised upon the column of mercury would cause it to 

 rise in one bulb, a/nd to descend correspondingly in the 

 other. This difference of level could not be observed by 

 any instrument ; but the upper portion of these bulbs is 

 filled with spirits of wine, and the spaces filled with the al- 

 cohol communicate by a glass tube with a single air-bubble so 

 adjusted as to occupy any position in this tube on the scale. 

 It follows then that any horizontal attraction which may 

 affect the level of the mercury, in however small a degree, 

 will force some alcohol through the tube, and carry this 

 air-bubble with it over the scale to an extent exactly 

 proportional to the horizontal attraction exercised ; and if 

 the cross-section or area of the tube is made 30,000 times 

 less than the area of the bulb, it follows that a rise of 

 mercury of ^.Wo f an i nc ^ in the bulb would cause the air- 

 bubble to move an inch \ and with such power of multiply- 

 ing the scale we have succeeded in obtaining indications 

 even of the weight of a single person passing from one side of 

 the instrument to the other. It thus renders sensible 

 the effect of our own weight in proportion to the enormous 

 weight of the earth. It might even be supposed that a 

 weighing-machine could be instituted, which being firmly 

 mounted on a solid foundation, would give you indications 

 of any passing weight in its vicinity \ but another appli- 

 cation which may be a useful one, is to measure the effects 

 of diurnal variations and the horizontal attraction exercised 

 by the moon and the sun, the force which produces the 



