340 



KNOWLEDGE 



September, 1910. 



attraction of the Moon." Furthermore, the forces 

 are inconceivably small, and, finally, they actually act 

 away from the Moon over one half of the Earth. 

 Remembering that the centre of the circle (Figure 1) 

 is the North Pole we can see that a drop of water 

 carried round the world from C through A D B and 

 back to C in twenty-four hours will for the first six 

 hours from C" to A feel itself urged eastwards : at A 

 it feels no force except a slight lift that cannot effect 

 any movement. From A to D it is urged westward 

 and again is left in peace when it gets to D. From 

 D to B the force on it is eastward, and from B to C 

 it is westward. And these forces act not only on 

 the water but on everjthing on or in the Earth. 



It has long been wished to obtain direct evidence. 

 b\' the help of some instrument, of these tide- 

 generating forces : the ocean alone shews them and 

 that is hardh' a convenient instrument for researcli. 

 Man\- attempts have been made, and in particular 

 there w as the pendulum constructed by Sir G. Darwin 

 and his brother, at Cambridge, which was of extra- 

 ordinarv delicac\-, and, in fact, it responded to such 

 tin\- forces that it was impossible to sa}' what parts 

 of its movements were to be laid to the charge of the 

 Moon. The worst trouble was the warping of the 

 whole solid crust every day in consequence of the 

 heating effect of the Sun. It is easily seen that if a 

 plummet be hung just over a mark by means of a 

 supporting scaffolding and the scaffold be then 

 tilted a little, the plummet will travel awa\' from 

 the mark. If the tilt is imperceptibly small it \\ ill 

 look as if the plummet had been pulled away by 

 some small attraction. This is called a spurious 

 deflection of the pendulum, and these tiny tilts of 

 the earth's crust are the great source of difficulty. 

 Professor Darwin found that his own \\eight as he 

 approached his instrument, made a perceptible 

 dimple in the earth's surface causing a tilt towards 

 himself which the pendulum duh' recorded. 



As already stated, Dr. Hecker has succeeded where 

 Darwin failed. Not onh' has his pendulum shewn 

 clearly that it swings twice each day under the 

 influence of the Moon, but indirecth- it also shews 

 that the whole continent of Europe rises and falls 

 twice a da\- much in the same way that a raft would 

 rise and fall w ith the tide. That simile will suggest 

 one at least of the difficulties to be contended with. 

 How could a sailor on the high seas stud\' the tides ? 

 How could he detect them at all ? So also no one 

 can measure directly the rise and fall of the earth's 

 surface : it must be done in an indirect way. 



Dr. Hecker's method was in principle the same as 

 Sir G. Darwin's. He measured the deflections of a 

 pendulum from a vertical ; and, as we have seen, the 

 pendulum wil move for two quite distinct reasons ; 

 (a) Any tilt of the whole surface on which the 

 instrument rests will produce an apparent deflection, 

 (6) Any attraction or drag horizontally will produce 

 a real deflection. Tilting will be produced not only 

 by the warping effect of the Sun's heat, but also by 

 any load placed in the neiglibourhood of the instru- 

 ment, A rise in the barometer means an extra load 



of air: the tilt so produced may amount to just 

 about as much as the lunar pull for which we are 

 looking. The addition of many thousands of tons of 

 water on the coast at high tide will produce a tilt far 

 inland such as the instrument \\ill quite clearly 

 record. We shall presently see how Dr. Hecker 

 contended with these difficulties. 



Dr. Hecker's success must be attributed to two 

 reasons. He carried on his work for more than two 

 years, and, secondh', he placed his instrument at a 

 depth of 80 feet in a disused well-shaft. The great 

 advantage of this position \\as that the warping due 

 to the Sun's heat was reduced to one-seventh of 

 what it would have been on the surface, but even so 

 its effect far exceeded that of the direct pull of the 

 Moon, which was the object of the search. 



The instrument that was used by Dr. Hecker at 

 the bottom of the well deserves some words of 

 description. It is called a horizontal pendulum, 

 because the swinging takes place horizontally like a 

 gate. It is in fact extremely like a well-hung gate 

 on two hinges. Such a gate will be so hung that it 

 closes itself by its own weight, and this is attained 

 In- very slightly inclining the hinge post inwards 

 towards the other. If, after completion the hinge- 

 l^ost settled a little and sloped outwards (at right 

 angles to the closed gate), clearly the gate would 

 have a new position of rest, and if the post were very 

 nearly vertical the gate would be sensitive to an 

 extremely small change of slope in the post, as long 

 as that change took place at right angles to the gate 

 when at rest. An additional slope towards the gate 

 would have no effect on it. For this latter reason a 

 pair of these pendulums must be used, their positions 

 of rest being at right angles to each other. Such a 

 pair would give records that practically come to the 

 same thing as the ordinary plummet, which can be 

 deflected in any direction, but they are infinitely 

 more sensitive. Dr. Hecker's "gates" are nine 

 inches long, but are as efficient as a simple vertical 

 pendulum as high as Mont Blanc. They enabled him 

 to read easily the thousandth part of a second of arc. 



These pendulums, in their well eighty feet deep, 

 automatically recorded their deflections during a 

 period of twent\--nine months from December 1902 

 to May 1905, and at the end of that period the 

 records were discussed. 



The greatest movement of which evidence was 

 found was quite unexpected and very annoying. The 

 instrument had been steadily tilting over more and 

 more in one constant direction throughout the entire 

 period. This was found to be due to settlement of 

 the whole well-shaft due to cracks in the masonr}-. 

 Fortnnateh', the movement was so regular that it 

 was possible to allow pretty accurately for it. 



The next obvious feature was a daily swing, and 

 this was attributed to the warping of the whole 

 continent by the heat of the Sun, and then among 

 several other minor disturbances was found a double 

 daih' oscillation due to the Moon. It will at once 

 be asked how a lunar disturbance could be distin- 

 guished from all the rest, and the answer is quite 



