August lb, 1917 
LAND & WATER 
17 
Also, if the acceleration is in the direction of motion the 
velocity increases even while the acceleration ^decreases.. 
Diagram V represents a wheel lying in a' horizontal position. 
Suppose tliat it is rotating in the same direction as the hands 
of a watch lying. on the. table face up. We wish to examine 
the effect of a steady upward pressure at A (out of the paper) 
and an equal downward pressure at B (into the paper) . That 
is to say, the effect of a force couple-acting about the axis 
CD. In tlie semicircle CAD the resultant force on every 
point will be upward. In the semicircle D B C the resultant 
force on every jxiint will be downward. As a point of the 
wheel goes from C through A to D it will have upward 
acceleration all the time, so that the maximum , upward 
velocity will be reached at D. • ' ' 
As a point of the wheel goes from D through B to C, it 
will have downward acceleration all the time, so that the 
maximum downward velocity will be reached at C. 
l*"rom .\ to D a point gains up-out velocity. 
D to B ,, loses up-out „ 
,, B to C ,, gains down-in ,, 
C to .\ ,, loses down-in 
Hence, though the forces act above the a.xis C D, the actual 
turning will be about the axis \ B. 
If Diagram V be held vertical with .\ as highest point, we 
shall be looking at the first experiment from the point of view 
of the operator. 
[N.B. — The rim of the wheel should be as heavy as possible 
so that 1h: spin of the wheel may have its full effect.] 
If the diagram be held with C as highest point we have 
experiment 2 from the same point of view. 
.\lthough this sort of treatment of a question in Rigid 
Dynamics may shock the consciences of some by its lack of 
rigidity, it is hoped that it achieves its purpose. This was to 
show how elementary considerations indicate the general 
character of the result. After all, when one has proved by 
Dynamics that a projectile will behave in a certain way there 
is interest in examining why it should do so. In conclusion 
I rnust again acknowledge my indebtedness to»Dr. Proud- 
man, though perhaps he would not like the reader to infer 
that he is responsible for these methods. 
The Green Spot 
By Morley Sharp 
THlv .Stiuadron-Conuiiandcr leant over tfie office 
table, his finger on the map, while a frown puckered 
his forehead. 
," Yes," said he to himself. " I think I've got it." 
He raised himself from his stooping positioii and slowly 
took out his cigarette-case. Then he walked to the window 
and looked out over the aerodrome. 
It was a quiet evening ; there was no breeze. The flag 
hung limply by its pole. The thunder of the guns had 
considerably lessened since noon. A couple of officers just > 
landed, walked leisurely over to the mess, carrying their 
heavy pilots' coats on their arms. The CO. looked at his 
wrist watch and, turning from the window, pressed an electric 
bell. An orderly entered ; 
" Tell Captain Briggs I want to see him." 
"Ah, Briggs," said the CO., "there's something here 
that wants attending to." 
Captain Briggs came and leant over the table beside the 
CO. while the latter traced on the map with his forefinger. 
The two lines of opposing trendies stretched parallel across 
the middle. The point to which the CO. directed attention, 
how-ever, lay beyond them -some ten miles on the German 
side. It was a small wcxjded holloy', lying between the bare, 
undulating folds of ground, and watered by a narrow stream. 
It was round in shape and its diameter might have Been a 
hundred meters. It lay not far from where four roads joined 
four main roads which convei'ge'd into one, the latter leading 
straight to the German lines. 
The C(J., having pointed out the green spot on the map. next 
laid his finger on the junction of the roads, and, looked at 
Captain Briggs. 
" There's been a lot of activity on those roads to-day," said 
he, ." everyone has reported something or other in the way 
of transport, and Parker repcjrts a whole column standing just 
tlierc, stationary, mind you," and he pointed to the portion of 
the road which was nearest the hollow. Captain Briggs 
nodded. 
" They'll be shifting their heavies further back, I'm think- 
ing," said the CO., " since we knocked out that lot," and he 
indicated a line of pencil crosses, midway between the four 
cross-roads and the (ierman trenches. "" 
Captain Briggs rolled the cigarette round in his mouth 
with his eyes on the green spot. 
" It's a fair-.sized target, anvhow," said he. 
" Kh ? " said the CO., " just so. It's a likely spot ' " 
He looked at his watch again and then out of the window 
at the sky. 
Second- Lieutenant Parker sat in his machine on the ground, 
his eyes on the "rev. counter," his hand on the switch. 
The Co. and Captain Briggs had finished instructing him and 
I'ad gone aside to escape the draught of the whirring propeller. 
Lieutenant Parker's duty as well as his map w^as befofe 
him, and, " throttling down," he waved his hand to the 
mechanics, who promptly pulled the chocks away that held 
fast the aeroplane's wheels. Slowly the machine began to 
move, then its pace quickened until it tore across the ground, 
ro.se up over the trees and circled round the aerodrome 
Lieutenant Parker continued his circles until he had risen 
to a height at which the aerodrome appeared to him about 
the size of a postcard. Then he made a bee-line for the green 
hollow, away beyond the enemy's lines. 
It was not many minutes before the four roads, looking 
much the same on the ground to Lieutenant Parker as they 
appeared on his map, were directly under him, and, closing his 
throttle ■ he nose-dived for several th»usand feet. Righting 
his machine he looked with the bornb-sight, until he stretched 
out his hand and released a bomb. As he looked over the 
side of his aeroplane to watch the effect, he saw a German 
machine, not very far below, coming up to meet him. 
The bomb burst on one of the roads, not far from the green 
hollow, and Lieutenant Parker could sec, far below, the hurried 
flight of men and transport along the four roads. He turned 
his machine and again took aim with the bomb sights. This 
time, as he released his bomb, he turned to his machine gun, 
to be prepared to combat his adversary, who was now almost 
on a level with him. 
\ hurried glance downwards showed him a bright yellow 
flame springing from the green hollow. .\t the same time, the 
concussion in the air jerked his aeroplane so much, that, had he 
not been securely strapped in, he must have been thrown out. 
A deafening roar down below shook the ground for miles 
around, and, at the aerodrome, the CO. and Captain Briggs 
speculated much on the task done by their young subordinate. 
But Lieutenant Parker's task was not yet over, for his 
aerial rival proved to be well-skilled in fighting. Round and 
round each other circled the tw'o aeroplanes and the machine 
guns spat out hundreds of rounds. Lieutenant Parker had 
nearly come to the end of his ammunition. He could sec 
his adversary preparing to fire. .\ couple of shots whizzed 
past him, close to his ear. He pulled the trigger of his gun, 
and the German pilot clapped his f|and to his neck. He was 
hit. In a flash the German machine had dived earthwards. 
Lieutenant Parker dived after him firing his last few rounds. 
But he had put his opponent out of action, and he felt that his 
work was done. So he righted his machine and made back 
for the aerodrome, to receive, on landing, the promise from the* 
CO. that he would be recommended for the M.C 
• * * • * 
.\nd that was how "one of our airmen successfully bombed 
a large enemv ammunition dump." 
