plates were made in like manner on that planet. Now Mars is an object which by reason of its smaller size is twice as difficult a test for rotation in twenty-four hours as Venus. The plates too did not happen to be so good. Nevertheless, on measurement they yielded a result within a twenty-fourth part of what we know to be the Mars day. For we know this time to within the hundredth of a second. Now in consequence of the smaller quantity to be measured an error of 55 minutes in the case of Mars corresponds to one of 31 minutes on Venus. To this precision, then, the day of Venus would have been determined had it been of twenty-four hours' duration.
Another test of like character was forthcoming in Dr. Slipher's spectrograms of the rotation time of Jupiter. Inasmuch as Jupiter's day at the equator is 9 hours 50.4 minutes long, while Jupiter's diameter is some twelve times that of Venus, the precision possible is here thirty times as great. Thirty-one minutes' error on Venus would mean about one minute for Jupiter. The spectrograms did even better than this. The known speed of rotation at Jupiter's equator is 12.63 km.; Dr. Slipher's spectrograms gave 12.62 km., or within half a minute of