386 
PHYSICS: C. G. ABBOT 
pheric transparency during the two or three hours necessary to the 
determination. It is this defect of the uniformity of the atmospheric 
transparency which requires us to make the measurements at so many 
stations in order to get a satisfactory result. 
Both the Mount Wilson and Calama observations indicate that the 
solar radiation was above its normal in the year 1918, having an average 
value of about 1.95 calories per square centimeter per minute. The 
mean value for many years was 1.93. 
ROTATING PROJECTILES FROM SMOOTH-BORE GUNS 
By C. G. Abbot 
Smithsonian Institution, Washington 
Read before the Academy, April 29, 1919 
In the late war much use was made of trench mortars. This kind of 
ordnance consists essentially of a smooth tube with a firing pin at the 
bottom. The projectile carries a shotgun shell at the rear. It is dropped 
down the smooth-bore barrel by the soldier and the firing pin explodes 
the primer on the shotgun shell. This in its turn explodes the charge of 
propellant which throws the shell into the enemy's lines. Many of the 
trench mortar shells tumbled end over end as they went over, but some 
were provided with fins which retained them in approximately steady 
flight. However the trajectory of these shells was by no means ideal. 
It occurred to me to try to secure sufficient rotation to produce steady 
flight by means of the turbine principle applied to the shells. Unfortu- 
nately the research was proposed to me only a few days before the armi- 
stice was declared so that by the time preliminary tests were made it was 
too late for the device to be of war service. However, the results 
appeared to be so promising that I have made numerous further experi- 
ments with a smooth-bore musket of 0.9 inch diameter which had seen 
service in the Civil War. 
The elongated ojival projectile was made in two parts in my experi- 
ments, the one a tough steel rear part chambered out to contain the pro- 
pellant, the other an aluminum nose provided with a steel plug at the 
rear for screwing into the steel part of the projectile. The steel plug 
serves the double purpose of closing the chamber and attaching it to 
the front part. With the aluminium part in front such projectiles are 
particularly well calculated for tumbHng, and invariably did so when 
fired in the usual manner. At the extreme rear of the steel part of the 
