m 
hy Christian Huygens. 
planation with which Wallis, who had at first entertained a doubt 
on this head, expresses himself perfectly satisfied. {Leyden MSS). 
The second class of clockworks. Fig. 4. had the axis to which 
the pallets were affixed in a horizontal situation, whilst the ba- 
lance TT moved in a vertical plane. In subsequently adopting 
this arrangement Fig. 2., it seepis that the vibrations of the pen- 
dulum now directly receiving the impulse of the pallets, became 
too large, and that it was in order to obviate this defect that 
Huygens suspended the pendulum from a thread between two 
curved brass-plates, which, by arresting it at a certain point of 
its course, prevented its going too far on either side. This de- 
parture from the original construction was not then published 
by Huygens ; but it must have occurred to him very soon after 
the publication of the first, whether with a view of adapting the 
new principle more easily to the then existing balance-works, or as 
a farther improvement of his own, (his activity and endeavours 
after perfection knowing no bounds, nil actum reputans si quid 
superesset agendum ) ; for, in a letter of M. Mylon, dated Paris, 
3Ist January 1659, vhis gentleman speaks of clocks, in which 
the axis lies horizontal, which, not having the pinion and wheel 
O, P, Fig. 1, are freed from certain inconveniences, but are 
liable to another, which,” he says, namely the inequality of 
the lengths of the vibrations, and consequently of the time, you 
have endeavoured to correct, by the addition of those two small 
pieces.” {Leyden MSS.) And Huygens himself, in a letter to 
Van Schooten, Professor at Leyden, of the 6th December 1659, 
says : You know, I think, that I employed in my clockworks 
two curved plates, between which the pendulum moved ; and 
that this was done, in order that the vibrations might all be 
made in equal times, as otherwise they would not be isochro- 
nous.” After he had used them for this purpose, he discovered, 
merit. The improved arrangement represented in Fig. 2. is taken from his work 
on the Theory of Pendulums, entitled Horologium Osoillatorium, which Huygens on- 
ly published in 1673, though, (as will appear in the text), it had occurred to him at 
a much earlier period, and was actually adopted soon after the discovery of the first. 
It must be observed, however, that the plates or cheeks, between which the pen- 
dulum is suspended, were not at first of the cycloidal shape, which he afterwards 
adopted and explained in this latter work, but were intended for a diiferent use, 
which is explained in the sequel. 
