6 SCIENCE PROGRESS 



it is necessary to magnify the effect. This is done by using 

 a horizontal pendulum, that is to say, a pendulum free to 

 swing about a nearly vertical axis. The deflexion of the 

 pendulum measures the force acting upon it. If the Earth 

 were absolutely rigid, the Moon would act upon the pendulum 

 with a certain force, as above. It is necessary to take numerous 

 precautions to shield the pendulum from disturbances, such 

 as those due to draughts, to the heating of the soil by the Sun 

 during the day and its cooling at night, even to the tilting 

 of the floor by the weight of the observer. All these difficulties 

 were overcome by Dr. O. Hecker, who installed two horizontal 

 pendulums in an underground chamber at Potsdam and re- 

 corded their movements during several years. On analysing 

 his results, he showed that the actual movement of the pen- 

 dulum is about two-thirds of what it would be if the Earth 

 were absolutely rigid. Hecker's measurement of the lunar 

 deflexion of gravity is a very remarkable achievement. It 

 recalls and evidently confirms the result obtained by ana- 

 lysing tidal observations to pick out the fortnightly tide ; and 

 it has itself been confirmed by another series of experiments 

 with horizontal pendulums carried out by Dr. A. Orloff at 

 Dorpat. It is important to note that the deflexion of the pen- 

 dulum or at least that part of it which is periodic in half a lunar 

 day keeps time with the Moon. 



The proper interpretation of these results is a matter of 

 some difficulty. The registering by the horizontal pendulum 

 of a deflexion less than that due to the Moon's force is evidence 

 that it is under the action of other forces which keep time with 

 the Moon ; and it is an immediate inference that these forces are 

 due to the deformation of the Earth by the Moon's tide-raising 

 force. This force alters very slightly the shape of the Earth, 

 elongating it towards the Moon and in the opposite direction 

 and flattening it all round at the places where the Moon is 

 near the horizon. The change of shape produces in the sup- 

 ports of instruments a slight tilt and consequently a horizontal 

 pendulum is subjected to a small force which may be described 

 as the "force due to tilting." It is easy to see that the force 

 due to tilting acts against the Moon's tide-raising force. But 

 this is not the only extra force which is exerted on the pendu- 

 lum. The elongation of the Earth in one direction, combined 

 with the flattening in all perpendicular directions, causes a 



