“Then it should be possible to calculate how long the cloud will be before it reaches us,” joined in Barnett. “I must say I don’t like the look of things. The way the cloud has increased its angular diameter during the last twenty years makes it look as if it’ll be on top of us within fifty or sixty years. How long do you think it’ll take to get a Doppler shift?”

“Perhaps about a week. It shouldn’t be a difficult job.”

“Sorry I don’t understand all this,” broke in Weichart. “I don’t see why you need the speed of the cloud. You can calculate straight away how long the cloud is going to take to reach us. Here, let me do it. My guess is that the answer will turn out at much less than fifty years.”

For the second time Weichart left his seat, went to the blackboard, and cleaned off his previous drawings.

“Could we have Jensen’s two slides again please?”

When Emerson had flashed them up, first one and then the other, Weichart asked: ‘Could you estimate how much larger the cloud is in the second slide?”

“I would say about five per cent larger. It may be a little more or a little less, but certainly not very far away from that,” answered Marlowe.

“Right,” Weichart continued, “let’s begin by defining a few symbols.”

Then followed a somewhat lengthy calculation at the end of which Weichart announced:

“And so you see that the black cloud will be here by August 1965, or possibly sooner if some of the present estimates have to be corrected.”

Then he stood back from the blackboard, checking through his mathematical argument.

“It certainly looks all right — very straightforward in fact,” said Marlowe, putting out great volumes of smoke.

“Yes, it seems unimpeachably correct,” answered Weichart.