Morning journal and courier., 1880-08-19
The Velocity of Light.
Measured by a New Method.

The N6lOClUy or LlSnCs
 Measured by a New Method.
 A correspondent of the New York Tribune gives the following account of an ingenious ap- paratus and it's use for the above purpose
 Professor Simon Newcomb, the eminent astronomer and superintendent of The Ma tional Almanac, has finished his new appara- tus for the ascertainment of the velocity of light and is making series of preliminary experiments to test it's parts.
 He was delayed several weeks in the work of construction by the difficulty experienced in getting small cogwheel of material pos sessing both strength and elasticity sufficient to receive power giving two and even three hundred revolutions second. Brass, steel and other metals were tried in vain, and suc- cess was only reached when, upon the sug- gestion of Alvan Clark, of Cambridge, Mass, the renowned maker of telescopes, raw-hide was adopted. The wheel is about an inch and half in diameter, and little thicker than silver dollar.
 Professor Newcomb has selected for his base of operations a point at Fort Whipple, on the Virginia side of the Potomac opposite Washington, where his apparatus has been set up in buildings erected for this purpose. Recently correspondent of the Tribune had an opportunity to see the apparatus in opera tion, and of hearing from the lips of Pro- lessor Newcomb an explanation of the prin-
  ed out.
 l Its most conspicuous parts are two brass \ tubes. about eight feet in length by about | two inches in diameter, placed horizontally l at right angles to each other, like the two l branches of the letter L, and resting at the | angle and at the outer ends upon low solid 1 columns of brick and stone. These tubes are | telescopes, differing from ordinary instru- ments only in respect of their great length ! and small diameter. One of them Is immov- 1 ably fixed upon it's bed of masonry, while the l other-being laid in plane about two inches 1 lower with it's 'object'' end upon 8 pivot ! swings by means of delicate screw to the l right or left through 8 radius of one or two degrees.
 At the angle formed by the line of the two 1 telescopes, and placed so as to be in the line of vision of both, stands small river of pe- culiar construction. It is square column of steel, five inches in length and two in di- l aneter, and plated upon it's four sides with | nickle It is mounted, top and bottom, upon pivots, so as to revolye horizontally, and it is l to this mirror that the rawhide cogwheel conveys it's power. Affixed to one of the l wheels which gives motion to the mirror is device for breaking electric current con- nected with recording clock, so that the number of revolutions of the mirror each second is automatically recorded. The pow CT to give the mirror it's revolutions is fur nished by steam engine built for the pur- pose, and placed in an adjoining building. few feet distant from the eye glass of the fixed telescope, and in direct line with the instrument, is placed helioscope, an appar- atus consisting of mirror mounted upon complicated bearings and moved by clock work, so that when wound up and adjusted it will follow the apparent motion of the sun rehecting it's rays for any desired length of time upon a given point The rays, or to speak scientifically, the waves of sunlight thrown from this instrument, are brought to focus by an intermediate lens upon the eye glass of the fixed telescope, through which they pass to the surface of the revolving mir TOT at the other end
 At the naval observatory in Washington, some 5000 feet distant from Fort Whipple across the Potomac, Gts exact distance will be ascertained by the coast survey before act ual work is begun) is placed fixed mirror, circular in form, three feet in diameter, and of such delicate degree of concavity that light thrown upon it from the revolving mir TOT at the fort is returned again and brought to a focus as near as may be upon it's start ing point. In other words, this mirror is a section of the inner surface of shell which if complete, would be three miles or more in diameter Of courss, to all appearance, it is perfectly fiat.
 Now, to understand the working of the ap- paratus let 1t first be supposed that a revolv- ing mirror is at rest The course of the suns rays will be follows: Striking the mirror of the heloseope, through the intermediate lens upon the eye-glass of the fixed telescope, through the instrument to one side of the re- volving mirror, thence to the concave mir TOT across the Potomac, by which they are returned to the revolving mirror, and are f. nally reflected through the movable telescope to the eye of the observer placed at the eye glass The point at which this telescope rest ts to receive the sun's rays after they have made their tortuous journey, the revolving mirror being at rest, is marked zero. Now apply the steam and set the revolving mirror in motion It sounds like the concentrated shriek of hundred terrified children. Give it 200 revolutions second, and it's four sides will throw 500 reflections of the suns rays across the Potomac to the mirror at the ob- servatory Nothing but the shriek indicates that the mirror is in motion it looks like small polished cylinder. The observer no longer catches the suns reflected rays through the movable telescope at zero, but swings his instrument to the left or right, &C., cording to the direction in which the mirror is made to revolve, until they again reach his eye. Reflections following each other with such enormous rapidity, appear 8 single reflection.
 This, then, is the whole process. It re- mains only to note the number of revolu- tions per second, as indicated by the record ing clock, and the degree of defection from it's normal position at zero given to the mov- able telescope, and work out the results as 'tsum'' in arithmetic. It will be seen that, while ray of sunlight is making its journey across the Potomac and back, between the revolving mirror and the fixed concave mir TOT, the former will have made some portion fsone of it's revolutions This portion is represented by the degree of dehection which it is necessary to give to the movable tele- scope to catch the rays upon their return 1 and second reflection from the revolving mir l TOT.