Page:Popular Science Monthly Volume 2.djvu/383

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marquable qu'un Astronome, sans sortir de son observatoire, en comparant seulement ses observations à l'analyse, eût pu déterminer exactement la grandeur et l'aplatissernent de la terre, et sa distance au soleil et a la lune, éléments dont la connaissance a été le fruit de longs et sensibles voyages dans les deux hémispheres,"[1] etc. (6me edition, p. 232).

It is evidently not the purpose of Theoretical Astronomy, then, to train faculties other than those employed in the higher mathematical investigations, and for this purpose no observatory is strictly necessary. But Physical Astronomy, as usually taught, confines itself to Descriptive Astronomy, and for that study it is undoubtedly necessary that students should have access (and a far freer access than they usually have) to instruments. To supply this want, "instruments of the largest size" have been too frequently supplied, so that it is possible that the student may contemplate the features of the moon, or the components of a coarse double star, through a 15-inch, nay, perhaps, a 20-inch object-glass, when perhaps a 4-inch telescope of Alvan Clark's make would serve the student's and the college's purposes fully. It used to be a saying of a celebrated American astronomer that "the price of telescopes increased directly as the cubes of the diameters of their object-glasses." If no higher ratio be the true one, it is evident that in the case supposed we have force misapplied, or not applied at all. It has become almost a reproach for a college not to own an equatorial of at least eight inches clear aperture; yet only consider how much of the best work of astronomy has been done with less apertures! Sir John Herschel and Sir James South executed a long and very refined series of measures of double stars with a much smaller instrument than the ordinary college equatorial, and much of Struve's best work is recorded as done with "the smaller instruments," and so on. It is not intended to advocate the use of poor instruments, nor specially of small ones, but to point out that the means should be adjusted to the object in view, and that no waste of power should be permitted. Again, Spherical Astronomy is taught in some colleges; and, in considering this branch of the subject, we are touching on its most useful portion. In nothing is a student's habit of accuracy more trained than in astronomical observation. There are minor points to be attended to each moment, and it is not until he ceases to be a pupil that he begins to be thoroughly at ease with even the simplest instrument. He has a running commentary of reasoning to make constantly, which is of the greatest value to him. He must constantly ask himself, while using his instruments, "If I do this or that, what will happen, and why?" Now, it is presumably to forward this

  1. It is a very noteworthy fact that an astronomer, without quitting his observatory, but by merely submitting his observations to analysis, could have determined exactly the dimensions of the earth, its oblate form, and its distance from sun and moon—data which have been obtained as the fruit of long and arduous voyages in both hemispheres.