388 
PHYSICS: C. G. ABBOT 
of the observatory enclosure there is no measurable lateral deflection of 
the projectile while in flight. Truly round holes are always left by the 
rotating projectiles. Very striking results were obtained in an experi- 
ment made in Virginia in the presence of a number of Ordnance Ofhcers. 
Two successive shots were fired at a target in one of which the projectile 
was made to rotate by means of the included charge and in the other of 
which the projectile was fired by powder outside of itself. In the one 
an excellent hit was made, leaving a true round hole, while in the other 
it happened that in the tumbling of the shell in the air it reached the 
target exactly broadside. The officers were naturally much impressed 
by this striking exhibit. 
As in most of the investigations we undertake, the same general idea 
had long before occurred to others. Patents for somewhat similar de- 
vices were granted as long ago as the Civil War. So far as I am aware, 
however, no one hitherto has attained so good a measure of success in 
applying the turbine principle to projectiles. 
MEANS OF MEASURING THE SPEED OF PROJECTILES IN 
FLIGHT 
By C. G. Abbot 
Smithsonian Institution, Washington 
Read before the Academy, April 29, 1919 
In the course of the experiments on rotating projectiles from smooth 
bore guns I desired to measure the speed of flight in some instances, and 
as I lacked the usual chronographic apparatus employed by artillerists 
the following substitute occurred to me and proved very satisfactory 
after a few trials. 
The projectile was fired obliquely across a horizontal beam of light 
reflected into the observatory from the siderostat. The solar beam was 
entirely cut off by a diaphragm at the wall of the observatory, except as" 
it passed through two round holes about 6 inches apart. The shot was 
fired through the left hand one of these holes. About 2 meters inside of 
the observatory another smaller diaphragm with a small aperture in it 
was placed opposite to the second or right hand hole. The direction of 
the gun was so arranged that the shot passed through the hole in this 
second diaphragm also. About a meter beyond the second diaphragm 
was placed a double tinfoil screen, the two tinfoils being separated by a 
sheet of cardboard and connected by wires respectively to a circuit con- 
