VELOCITY AND PRESSURE INSTRUMENTS. 
375 
Mean Errors.—Simultaneous Observations. 
1st Experiment. 
April 21, 1875. 
secs. 
secs. 
Disc 1 mean error . 
*000006 
Disc 5 
mean error 
. -000004 
2 
*000004 
, 6 
n 
. *000002 
3 
*000003 
„ 7 
a 
. *000005 
II 
4 
•000002 
„ 8 
n 
. *000002 
2nd Experiment. 
April 21,1875. 
secs. 
secs. 
Disc 1 mean error . 
. *000004 
Disc 5 
mean error 
. -000003 
II 
2 
. *000006 
„ 6 
„ 
. -000005 
II 
3 
•000005 
„ 7 
„ 
. *000003 
II 
4 
. -000007 
„ 8 
a 
. *000000 
April 22,1875. 
secs. 
secs. 
Disc 1 mean error . 
. *000004 
Disc 5 
mean error 
. *000003 
„ 
2 
. *000004 
„ 6 
n 
. -000006 
„ 
3 a . 
. *000005 
„ 7 
a 
. *000006 
n 
4 a .. 
. -000005 
„ 8 
n 
. *000000 
April 28, 1875. 
secs. 
secs. 
Disc 1 mean error . 
•000008 
Disc 5 
mean error 
. -000002 
„ 
2 
*000006 
„ 6 
a 
. *000003 
„ 
3 
*000004 
„ 7 
n 
. 000011 
n 
4 
•000002 
„ 8 
n 
. *000003 
The errors in none of these observations exceed a few millionths of a 
second, though, as before stated, they include any variation in the time of 
cutting the wires. The mind is quite incapable of appreciating the minute 
intervals of time of which we are treating, but the extreme accuracy of the 
instrument will be apparent when we consider that the one-millionth part of 
a second is about the same fraction of a whole second as a second is of a 
fortnight. 
The gun shown in Eig. 3, Plate Y., is an 18-ton 10-in. rifled M.L. gun, 
having a length of bore of 145*5 ins. 
It was tapped at three places in the powder-chamber, to receive crusher- 
gauges. These holes could be closed with solid plugs when not required. 
The cutting plugs belonging to the chronoscope were eighteen in number, 
and were fitted into holes at various intervals along the bore, as shown in 
Plate Y. When not required for the chronoscope these holes could be 
used for crusher-gauges, or were closed with solid plugs. The construction 
of the cutting plugs has been already described. The holes for the cutting 
plugs at the breech end of the bore were so arranged that the front edge of 
the projectile was flush with one of them with eacli of the different charges 
employed. The projectiles used in these and similar experiments are cast- 
iron cylinders, having a windage of only 0*01 in* 
49 
