ELASTIC FORCES OF AQUEOUS VAPOUR, 587 
derably : however, considerable errors of measurement might 
arise, if the meniscus in the tube mm and the meniscus in the 
tube p g were read off successively with the same cathetometer. 
Besides, this operation would be impossible when the difference 
of level surpassed 1 metre, which is the greatest length of the 
range of our cathetometers: therefore two cathetometers were 
used at a time. Lach of these instruments followed one of the 
menisci; and it thus became easy to observe the perfectly cor- 
responding positions of the two menisci, choosing that moment 
when the extent of the oscillations was very small (one-tenth 
of a millimetre) about one point of equilibrium. The observa- 
tions were made simultaneously by two observers, on a signal 
being given by one of them. The two cathetometers were regu- 
lated to the same fixed point traced upon the tube p g, very near 
the position into which the meniscus was constantly brought. 
At the same time there were traced on the tube m a, at inter- 
vals of 0™-90, fixed points, the distances between which had been 
read off a great many times by means of the cathetometer, Be- 
tween these different determinations of one and the same length, 
more than 3 metres, there was never found a difference amounting 
to one-tenth of a millimetre ; which proves sufficiently the exacti- 
tude of the mode of measurement. [I still wished to ascertain, 
by direct measurements, if the scales of the two cathetometers 
were identical, and what degree of precision could be attained 
by this method of observation. For this purpose I placed the 
two instruments opposite to each other, in such a position that 
by means of the telescope of the one the divisions of the other 
could be read in the most distinct manner, and vice versd. I 
then took, proceeding from 5 to 5 centimetres at a time, the 
divisions of one of the instruments by means of the micrometric 
apparatus of the other. In this manner! obtained in no partof the 
whole range of 1 metre a difference amounting to one-twentieth 
of a millimetre. To obtain alike precision in the measurements, 
it is evident that the instruments ought to be constructed in the 
most perfect manner; the telescopes should not have a too 
great focal length (0™30), and in particular the levels of the te- 
lescopes should be extremely delicate. The levels of the cathe- 
tometers of M. Gambey indicate inclinations of one second. 
The verniers give immediately the one-fiftieth of a millimetre, 
and the ;} 5th is easily estimated. 
The mercurial thermometers which I employed in these ex- 
periments were ordinary thermometers with straight stems ; one 
