i5o 



KNOWLEDGE & SCIENTIFIC NEWS. 



LOcTOBhR, 1906. 



wards used by Cavendish (1798), by Reich (1836), and 

 by Baily (1842) in their famous gravitation experiments 

 were nothing more nor less than torsion balances, even 

 though bitilar silk fibre suspensions were in some cases 

 used instead of a single wire suspension to give definite 

 directive tendency to the pendulum rod, by limiting its 

 freedom (see Fig. 5). 



'I'hc advantage of defmite directive trndrncy stained 

 bv susjX'nsion, whether unililar or bililar, and whether 

 In- cord, wire, fibre or other material, was only gained 

 at the cost of restoring in some degree the disadvantage 

 of having to lift to a very slight extent against earth's 

 \ertical downward attraction the weight of the pendu- 

 lum rod with its two balls. For torsiim implies twis- 

 ting, and consequent shortening or lengthening of the 

 suspension, and raising or lowering of the rod, etc. 

 .And granting that in alternate excursions of oscillation, 

 about a mean direction line of rest, the raising and 

 lowering inxofvcd might be equal and opposite in 

 amount, and neutralise each other, yet the fact would 

 remain that the path of each ball would tend to have a 

 slightlv spiral form, and not be strictly horizontal. 



With those horizontal pendulums pairs of large 

 attracting masses were u.sed, and tbe.se were so placed 

 near the pendulum balls that their attractive forces 

 were exerted at the same time, so as to produce a com- 

 bined rotational effect in the same direction in the ap- 

 proximately horizontal plane. That rotational effect, or 

 lateral pull, with reference tO' the delinite direction line, 

 or mean line of rest, was calculated from the number cf 

 oscillations, the weights and distances of the attracting 

 mas.ses invohed, and the times, by using Coulomb's 

 formula, etc. 



.\ ^■ery full account of the latest of those experi- 

 ments, and reference to- others made previously, will 

 bv' found in \'ol. 14 of the " Memoirs of the Royal 

 .\stronomical Society, 1843." In all ca.ses of such 

 experiments on gravitation, with horizontal pendulums, 

 " anomalies " of attraction were noticed, resultint^ in 

 a " march " of scale readings, or " reversals " of 

 direction, etc., for which the observers were un- 

 able tO' account. Such anomalies were discus.sed by 

 Poisson (1834), Plana, John Herschel, Heme, and 

 others mathematically and otherwise. Grant's " Hi.s- 

 tory of Physical Astronomy " may be consulted for 

 further reference^thereon. 



(C) Inclined pendulums. — These are wronglv called 

 horizontal pendulums usually, and apparently for iio 

 better reason than that the rod used is sometimes ap- 

 proximately horizontal, and is intended to be u.sed ;n 

 the investigation of force exerted in a horizontal plane. 

 But they do- not oscillate in a horizontal plane, thev are 

 not as a rule even intended or designed to- do so. Im- 

 plicitly or expressly, others are designed so that thev 

 are not horizontal, and can only be properly u.sed in 

 inclined planes. 



The absurdity of classing these pendulums with those 

 of the class which contains Coulomb's and Cavendish's 

 jXMidulums w-ill best be appreciated by observing the 

 essential distinction which exists between them. The 

 Coulomb and Cavendish class, in addition to the char- 

 .icteristics already noticed, involves two weights on one 

 rod, and the earth's downward attraction upon one of 

 tho.se weights is balanced by the similar attraction upon 

 the other, as at the opposite end of a lever ha\ ing ,ts 

 fulcrum in the middle or where the suspension line is 

 attached, and where the centre of oscillation is supposed 

 to remain constant in position, but in practice cannot 



and does not do so. Whereas the inclined pendulums 

 ha\e a weight only on one end of a rod, and in con- 

 sequence of having no counterbalancing weight at the 

 other end to neutrali.se the earth's attraction upon it, 

 various different devices have been used to effect that 

 neutralisation otherwi.se than by using a second weiglit, 

 and subject to the retention of a very minute iinneutra- 

 lised fraction of its own weight merely for the sake of 

 imparting delinite directive tendency by inclining the 

 pandulum specially for that purpose. Moreover, while 

 those in the former class never have a constant centre 

 of oscillation, tho.se in the inclined class may have, and 

 in sO'me instances practically have. 



Henglcr's Pendulum (1832), appears to be the 

 first inclined astronomical pendulum known in modern 

 times. His account of same I found this year (1906) 

 in Dingier' s Polyleehnisc/ies Journal, 1832, printed in Lhe 

 old fierman characters. I know of no translation ever 

 having been published in the English language, and 

 have had to make a translation sjx-ci.-dlv for my own 

 satisfaction. 



His rod was suspended by a short cord or wire from 

 a point in the ceiling and so that the length of the rod 

 was divided into two very unequal portions, as a lever 

 having unequal arms. .\t the outer end of the longer 

 arm he hung a small weight, and at the outer end of 

 the short arm he tied one end of a, long cord (or wire), 

 w hich extended downwards to and had its lower end 

 fastened to the floor at a point almost vertically below 

 the point aforesaid in the ceiling. He arranged the 

 rod almost horizontally by adjusting the length of the 

 long cord going to the floor and the function of that 

 long cord was primarily to supply by its tension a 

 counterbalancing force against the weight on the other 

 arm. So he avoided the u.se of a .second weight, and it 

 is ob\ious that the small weight at the outer end of the 

 long arm might oscillate in the arc of a nearly horizon- 

 tal circle, about the centre of oscillation, which w-as at 

 a point intermediate between the two points of attach- 

 ment of the cords aforesaid, to the rod (see Fig. 6). 



Of course, " torsion " effect was a factor, as well as 

 gravity, in determining the direction that the rod would 

 tend to assume at rest, but he clearly saw the impor- 

 tance of setting his pendulum to oscillate in an inclined 

 plane, for he discus.ses its angle of inclination, etc., and 

 designed his pendulum to oscillate therein. He set the 

 rod so as to rest in the meridian at noon on a dav of 

 new moon, then closed the room, and ob.served tb.e 

 oscillations of the weight by means of a powerful micro- 

 scops from outside the room. 



He devised also a similar pendulum, having a 

 separate stand, so obviating, if required, the necessity 

 of direct attachment to the building in which experi- 

 ments were to be made. 



With one of his pendulums he claims to ha\e ob- 

 served lunar attraction effects, and with another, earth 

 rotation efl'ects. His statements as to the diurnal 

 variation which he says he observed are very inter- 

 esting. More will be heard about such matters 

 pre.sently. He was a pupil of Gruithuisen (vide 

 Safarik, \'ol. 46, Phil. Mug., 1873). Both master and 

 l)upil ha\e been gratuitously villified since their 

 decease by very plausible people, who had ends of 

 their own to serve, by belittling others that thev bv 

 contrast might appear great. 



Hengler was a genius, and by dispensing with the 

 nec;'ssity of using a second or balancing weight, as 

 used in the class of so-called " horizontal " pendulums. 



