50 Messrs. Ayrton and Perry on the Determination 



which is thus passed sub silentio is not even mentioned." But 

 with us ho is more decided in his expressions ; for he says 

 that our having only referred to the names of Clairault, Kater, 

 and Borda is carrying " originality in experimental research 

 too for — even to the very verge of decency." But we were 

 not writing a history of pendulum-observations, much less of 

 geodetic survey ; all we desired to do in our short paper was 

 to give an account of certain experiments our students had 

 made to determine (as we believe for the first time) the 

 acceleration due to gravity in the capital of Japan. As a 

 matter of fact, the names of the students who assisted us, 

 given at the end of our paper, are the only names we need 

 have mentioned ; and we quite fail to see the indecency of 

 our not having expanded a short account of a modern experi- 

 ment into an historical treatise on geodetic survey. 



V. Permit us to assure Major Herschel that the joint work- 

 ing of ourselves and our students in the laboratory for several 

 years has not been of such a nature as to require their 

 " demanding to know why " certain experiments resulted as 

 they did. The reversible pendulums we employed were such 

 as are usually found among the collection of physical appa- 

 ratus at a university. Although costing several pounds, they 

 were in workmanship probably much inferior to those used 

 for very accurate pendulum-observations; and under such cir- 

 cumstances we are sure that better results can be obtained 

 with a long wire pendulum. 



Pianoforte-wire and a ball of cast iron would enable a pen- 

 dulum 200 feet long to be easily experimented with in any 

 high chimney or tower. If the ball had more than 3 inches 

 diameter, air-friction would not practically exist, and the 

 correction for buoyancy and for air-inertia could be accurately 

 made. With a complete amplitude of 30 centimetres (the 

 average swing backwards and forwards that we employed), 

 the arc-correction would be too insignificant to mention, the 

 average velocity of the ball would be less than two centi- 

 metres per second, so that centrifugal force would introduce 

 no lengthening of the wire ; indeed that would be the case 

 even with the larger arc of a single vibration, 30 centimetres, 

 which we used in our paper when calculating the maximum 

 possible effect of centrifugal force. The length of the wire 

 could be measured with great accuracy; alterations due to 

 change of temperature might be accurately allowed for ; and 

 slight discrepancies as to the position of the centre of gravity 

 of the ball would not sensibly affect the answer. The arc of 

 such a pendulum, after swinging for some days, would pro- 

 bably not have become less than 2 or 3 centimetres, quite 



