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race of the vehicle。 You know the rule： two objects cannot occupy the same space at the same time。〃
〃That's elementary。〃
A curt nod。 〃It is so elementary that our engineers overlooked it； and nine men died when the vehicle returned to its point of origin； its precise second of launch； and attempted to occupy the same space。〃 His voice dropped。 〃Chaney； the most dreadful sight I have ever seen was the crash of an airliner on a Dakota hillside。 I was with a hunting party less than a mile away and watched it fall。 I was among the first to reach the wreckage。 There was no possibility of anyone surviving…none。〃 Hesitation。 〃The explosion in our laboratory was the second worst sight。 I was not there…I was in another building…but when I reached the laboratory I found a terrible repetition of that hillside catastrophe。 No man； no single piece of equipment was left intact。 The room was shattered。 We lost the engineer traveling with the vehicle and eight others on duty in the laboratory。 The vehicle returned to the exact moment； the exact millisecond of its departure and destroyed itself。 It was an incredible disaster； an incredible oversight… but it happened。 Once。〃
After a space； Seabrooke picked up the thread of his recital。 〃We learned a bitter lesson。 We rebuilt the laboratory with thicker； reinforced walls and we rebuilt the vehicle； we programmed a new line of research accenting the safety factor。 That factor settled itself at just sixty…one seconds； and we were satisfied。〃
Chaney said： 〃They've been counted for me； again and again。 I'll lose a minute on every trip。〃
〃A passenger embarking for any distant point； you； will leave at twelve o'clock； let us say； and return not sooner than sixty…one seconds after twelve。 The amount of elapsed time in the field will not affect the return； if you stayed there ten years you would return sixty…one seconds after you launched。 If we could not be absolutely certain of that we would close shop and admit defeat。〃
〃Thank you；〃 Chaney said soberly。 〃I like my skin。 How are you protecting those men now？〃
〃By reinforced walls and remote observation。 The engineers work in an adjoining room but five feet of steel and concrete will separate you。 They operate and observe the TDV by closed circuit television； indeed； they observe not only the operations room itself but the corridor to it and the storeroom and fallout shelter： everything on that level of basement。〃
Curiously： 〃How do you really know the vehicle is moving？ Is it displacing anything？〃
〃It does not move； does not travel in the sense of passing through space。 The vehicle will always remain in its original location； unless we choose to move it elsewhere。 But it does operate； and in operation it displaces temporal strata just as surely as those people in the pool are displacing water by plunging into it。〃
〃How did you prove that？〃
〃A camera was mounted in the fore of the vehicle； looking through a port into the operations room。 A clock and a day…calendar hang on a wall in direct line of sight of that camera。 The camera has not only photographed past hours and dates but has taken pictures of the wall before the clock was placed there。 We know the TDV has probed at least twelve months into the past。〃
〃Any effect on the monkeys？〃
〃None。 They are quite healthy。〃
〃What have you done to prevent another accident…a different kind of an accident？〃
Sharply： 〃Explain that。〃
Chaney said carefully： 〃What will happen if that machine probes back into the past before the basement was dug？ What will happen if it burrows into a bed of clay？〃
〃That simply will not be allowed to happen；〃 was the quick reply。 〃The lower limit of displacement is December 30； 1941。 A probe beyond that date is prohibited。〃 The Director emptied his glass and put it aside。 〃Chaney； the site has been carefully researched to determine a lower limit； every phase of this operation has been researched so that nothing is left to chance。 The first building on the site was a crude structure resembling a cabin。 It burned to the ground in February； 1867。〃
〃You went back that far？〃
〃We were prepared to go farther if necessary； we had access to records dating back to the Black Hawk war in 1831。 A farmhouse with a basement was built on the site during the summer of 1901； and remained in place until demolition in 1941 when the government acquired this land for an ordnance depot。 It has since been government owned and occupied； and the site remained vacant until the laboratory was built。 The engineers were very careful to locate that basement。 Today the TDV floats in a sealed tank of polywater three feet above the original basement floor； in a space that could have been occupied by nothing else。 We even pinpointed the former location of the furnace and the coal room。〃
〃And so the deadline is 1941？ Why not 1901？〃
〃The lower limit is December 30； 1941； well after the date of demolition。 The safety factor above all。〃
〃I'd like to see that tank of polywater。〃
〃You will。 It is necessary that you bee quite familiar with every aspect of the operation。 Have you been visiting the doctor for your physicals？〃
〃Yes。〃
〃Have you had weapons training？〃
〃No。 Will that be necessary？〃
Seabrooke said： 〃The safety factory； Chaney。 It's wise to anticipate。 The training may be wasted； but it's still wise to prepare yourself in every way。〃
〃That sounds pessimistic。 Wasted in what way？〃
〃Excuse me； you've been out of the country。 All weapons for civilians will probably be prohibited in the near future。 President Meeks favors that； you know。〃
Chaney said absently： 〃That will please the Major。 He doesn't believe civilians have enough sense to point a gun in the right direction。〃
He was looking across the pool。 Katrina had left the water and was now perched on the tiled rim of the opposite side； freeing her hair from the confines of a plastic cap。 Arthur Saltus was as close as their two wet suits would permit； but none of the loungers about the pool were staring at him。 Two other women in the water weren't drawing half the attention…but neither were they as exposed as Katrina。 Military codes extended to the swimming pool whether WACS liked it or not。
Chaney continued to stare at the woman…and at Saltus hard by…but a part of his mind dwelt on Gilbert Seabrooke； on Seabrooke's matter…of…fact statements。 He thought about the machine； the TDV。 He tried to think about the TDV。 Every effort to visualize it was a failure。 Every attempt to understand its method of operation was a similar failure…he lacked the engineering background to prehend it。 It worked： he accepted that。 His own ears told him that every time they rammed through a test。
Drawing an enormous amount of power and piloted by a remote guidance； the vehicle displaced…what？ Temporal strata。 Time layers。 The machine didn't move through space； it didn't leave the basement tank； but it… or the camera mounted in the nose…peered and probed into time while photographing a clock and a calendar。 Soon now； it would transmit humans into tomorrow and those humans were expected to do more than merely look through the nose at