"To be safe. Remember the acceleration the Lark will be capable of, and also that on some other worlds, which we hope to visit, this needle will weigh more than it does here."
"That's right, Mart, I never thought of that. Anyway, we can't be too safe to suit me."
When the compasses were done and the power through them had been adjusted to one-thousandth of a watt, the lowest they could maintain with accuracy, they focused each instrument upon one of a set of most carefully weighed glass beads, ranging in size from a pin-head up to a large marble, and had the beads taken across the country by Shiro, in order to test the sensitiveness and accuracy of the new instruments. The first test was made at a distance of one hundred miles, the last at nearly three thousand. They found, as they had expected, that from the weight of the object and the time it took the needle to come to rest after being displaced from its line by a gentle tap of the finger, they could easily calculate the distance from the compass to the object. This fact pleased Crane immensely, as it gave him a sure means of navigation in space. The only objection to its use in measuring earthly distances was its extreme delicacy, the needle focused upon the smallest bead in the lot at a distance of three thousand miles coming to rest in little more than one second.
The question of navigation solved, the two next devoted themselves to perfecting the "X-plosive bullet," as Seaton called it. From his notes and equations Seaton calculated the weight of copper necessary to exert the explosive force of one pound of nitro-glycerin, and weighed out, on the most delicate assay-balance made, various fractions and multiples of this amount of the treated copper, while Crane fitted up the bullets of automatic-pistol cartridges to receive the charges and to explode them on impact.
They placed their blueprints and working notes in the safe, as usual, taking with them only those notes dealing with the object-compass and the X-plosive bullet, upon which they were still working. No one except Shiro knew that the original tracings, from which the blue-prints had been made, and their final, classified notes were always kept in the vault. They cautioned him and the three guards to keep a close watch until they returned. Then they set out in the biplane, to try out the new weapon in a lonely place where the exploding shells could do no damage.
They found that the X-plosive came fully up to expectations. The smallest charge they had prepared, fired by Crane at a great stump a full hundred yards away from the bare, flat-topped knoll that had afforded them a landing-place, tore it bodily from the ground and reduced it to splinters, while the force of the explosion made the two men stagger.
"She sure is big medicine!" laughed Seaton. "Wonder what a real one will do?" and drawing his pistol, he inserted a cartridge carrying a much heavier charge.
"Better be careful with the big ones," cautioned Crane. "What are you going to shoot at?"
"That rock over there," pointing to a huge boulder half a mile away across the small valley. "Want to bet me a dinner I can't hit it?"