The first batch of machines arrived in France towards the end of April.
The Mark IV. Tank was an improved Mark I., and did not differ very materially from its predecessor in design.
These were, briefly, the principal improvements:
First, its armour was of a special steel which was impervious to the German armour-piercing bullet.
Secondly, the sponsons were of a better pattern. In the Mark I. they had to be completely unshipped whenever the Tank was moved by rail; in the Mark IV. they were so constructed that they could be “pushed in” sufficiently for railway transport.
Thirdly, a new and heavier design of track rollers and links was introduced.
Fourthly, danger from fire was reduced by the petrol tank (protected, of course, by special armour-plates) being outside and at the back of the machine.
There were also other minor improvements in armament, and the total weight of the Tank was slightly reduced.
Such was the weapon which was to be first tried at Messines, and such was the unit which was at the same time to make its début.