hy Christian Huygens. \ 211 
fiour, with its smallest divisions, (^imurle^^ from noon, or from 
the- setting of the sun. Of which he says, “ I possess mca-^ 
sures of time {mesuratore del tempo) such, that if one constructs 
four or six similar instruments, one will find, as a proof of their 
accuracy, that the times which they .measure and indicate {tempi 
da quelli mesurati e mostrati^) do not differ ^ one second, not 
only in an hour, but a day, a month ; so uniform are these 
clocks {oruoli,) fully {pur troppd) astonishing to observers of 
celestial phenomena and motions ; the more because the con- 
struction of those instruments {mstrumenti) is very easy and 
simple, and little subject to those external hindrances which 
other instruments devised for the same purpose are liable to.’’ 
The word oruoli (horologes,) which here occurs, must be parti- 
cularly attended to ; for though it suggests to us, and did even 
suggest at that time the idea of an instrument indicating the 
time by the regular motion of the hands, it appears from Ga- 
lileo’s own description of them, in a subsequent letter, written in 
June of the same year, that he meant something quite different 
from it. After explaining the chief principles of this theory 
of the pendulum from his JDiahgi de Motu^ (which were then 
printing at Elzevir’s,) he adds : “From these true and well 
established principles, I derived the construction of my reckon^ 
ers of time {numeratore del tempo) ^ and I use not a weight 
suspended by a thread, but a pendulum {pendole) of some 
ponderous and more solid stuff, {de materia solida e grave ^) 
as brass or copper : I make the pendulum in the form of a sec- 
tor of twelve or fifteen degrees, its seniidiameter of two or three 
palms, (between sixteen and twenty-four inches,) the larger it 
is the more easy will it be to be employed {con minor tedio se 
gli potra assistere). I make this sector thick in the semidia» 
meter of the middle, and becoming thinner towards the edge, 
by which means I obtain a cutting side, which will enable it to 
overcome, as much as possible, the resistance of the air, which- 
alone retards its motion. In the centre is a hole through which 
an iron axis passes, like that of a balance, with a sharp edge 
below, resting on two supports of bell-metal.” “ It will be ne- 
cessary,” he farther adds, “ in order to continue its motion, that 
an assistant shall, from time to time, give it a pretty strong im- 
pulse, {im impulso gagliar do), to restore the length of its vibra- 
