SCIENCE. 



263 



volving mirror, and the fixed mirror at a distance of about 

 2000 feet. By this arrangement, with a speed of rotation 

 of 257 turns per second, he obtained a deflection of 115 

 millimeters ; whereas, Foucault using a speed of 400 

 turns per second, and causing the light to traverse a 

 distance of 20 meters, had obtained a deflection of 8 

 millimeters. The revolving mirror was driven by a tur- 

 bine-wheel operated by a blast of air. Its speed, which 

 was measured by an electric tuning-fork, was readily 

 adjusted by a stop-cock, and the deflection was measured 

 by a micrometer. 



Mr. Michelson gives a most careful discussion of the 

 errors of his constants, including the determination of the 

 value of the micrometer screw, the rate of vibration of 

 the tuning fork, etc., and concludes with the considera- 

 tion of several "objections " which have been suggested 

 from time to time. 



The final value for the velocity of light in vacuo is 

 299944 ±51 (in air, 299864), or, in round numbers, 

 299940 kilometers per second = 186380 miles per second, * 

 the remarkably small error, + 51 kilometers, being com- 

 posed of the total constant error in the most unfavorable 

 case, and the probable errors of observation. This quan- 

 tity. ±51 kilometers, cannot be said to express the prob- 

 able error of the determination, in the ordinary accept- 

 ance of the term ; combining, as it does, probaole errors, 

 strictly speaking, and estimated constant errors. 



These experiments were made by Master Michelson at 

 the Naval Academy, Annapolis, at private expense, and 

 to him the entire credit is due. A new determination of 

 the velocity of light, embodying essentially the same ar- 

 rangement, but with more elaborate and expensive appa- 

 ratus, is now being made under official auspices by Prof. 

 Newcomb, Superintendent of the Nautical Almanac. 



It is probable that in this way the most accurate value 

 of the solar parallax, so essential to astronomy, can be 

 deduced. 



Astronomical Memoranda. —^Computed for the 

 meridian of Washington, D. C, November 29, 1880) : 



H. M. S. 



Sidereal time 05 mean noon 16 35 50 



Equation of time .- n 15 



mean noovi. following apparent noon. 



The Sun is 21 ° 39' south of the equator, at meridian 

 transit, and will continue to move south until Decem- 

 ber 21. 



The Moon reached its last quarter on November 24d. 

 8h. 47m. It does not come to the meridian until 10 A.M. 

 of November 30. 



Mercury was in inferior conjunction, November 23, 

 and is not now visible to the naked eye. It precedes the 

 sun by about 52 minutes, and is five degrees farther 

 North. 



Venus is plainly seen in the southwest ; a short time 

 after sunset ; following the sun by 2h. 33m., and gradually 

 increasing the distance. Its declination is 24° 43' south. 



Mars is at present too close to the sun for observa- 

 tion. 



Jupiter, though gradually growing fainter, is still the 

 most brilliant object in our eastern sky at evening. It 

 passes the meridian at 8 P.M., at an altitude of 54 above 

 the southern horizon. Its more exact position at that time 

 is: Right Ascension, oh. 37m. 43s.; declination, +2° 28'. 



Saturn, less brilliant than Jupiter, is, notwiths'anding, 

 equal or superior to the larger planet in point of interest. 

 It isreaddy found about 13' E. by N. of Jupiter, present- 

 ing a good view of the southern side of its rings. 



Uranus is in right ascension nh. om. 52s. declina- 

 tion + 7" 9'. 



* Note. — Foucault's experiments gave the velocity as 185300 miles per 

 second. 



Neptune is in excellent position for observation, reach- 

 ing the meridian at about 10 P. M. It was in opposition 

 on November 4, and may now be found in Right Ascen- 

 sion 2h. 41m. 27s. declination +13° 46'. 



A new 10 in. equatorial, with an object glass by 

 Merz, has been presented to the Geneva Observatory by 

 its director, Prol. Emil Plantamour. It is to be devoted 

 to observations of the major planets and their satel- 

 lites, of parallax of stars, and ot double stars, with oc- 

 casional observations of minor planets. 



Dr. Schmidt has made a new determination of the 

 time of rotation of Jupiter upon its axis, from observa- 

 tions in 1879 and 1880, of the red spot upon its disc. 

 His preliminary discussion gives for the time of rotation 

 9h. 55m. 34.42s. 



In a letter to Nature, dated October 2, Prof. Picker- 

 ing, of Harvard College Observatory, announces that the 

 period of Ceraski's new variable star is probably 2.5 

 days, instead of 5 days, as previously published by 

 Scnmidt. It is especially remarkable for the rapidity cf 

 change during part of its period. The total variation is 

 from about the 6.7th to the 10th magnitude. The ap- 

 proximate place for 1881 is, R. A. oh. 51m. 48s. Dec. 

 + 81 I4'.i. W. C. W. 



Washington, D. C, Nov. 23, 1880. 



MICROSCOPICAL COLLECTIONS IN FLORIDA.* 



By Dr. C. C. Merriman. 



It has been my fortune during the past two Winters to 

 spend a few weeks in the regions of Central Florida. Lake 

 Harris is the most southern and the most beautiful of the 

 cluster of lakes which forms the source of that exceedingly 

 picturesque river, the Ocklawaha. With high hanks, and 

 surrounded by a belt of hummock land as rich as any that 

 Florida affords, this lake is becoming settled upon, and its 

 lands are fast being taken up by enterprising southerners 

 for orange-groves and pine-apple plantations. The so- 

 journer will find the society of this lake-settlement intelli- 

 gent and hospitable beyond anything that would be ex- 

 pected in so new and pioneer a country. The vegetation 

 of this almost tropical region is so full of interest to the 

 microscopist, and the causes conducing thereto so peculiar, 

 thai I have thought them deserving of especial mention and 

 illustration. 



The absence, or at least the rarity of frosts injurious to 

 vegetation in these lake districts, gives the longest possible 

 season for the growth and maturity of such organs as are 

 best, or especially, adapted to the exigences of Florida 

 plants. There is a period of rest, usually comprising about 

 the three Winter months, after which vegetation takes up 

 and continues its growth again as if there had been no 

 period of interruption ; so that practically there is a contin- 

 ous development of plant lite, whether annual or perennial, 

 from birth to death. 



The soil of Florida, as of all the South-Atlantic sea- 

 board, is sandy and naturally barren. No polar glaciers 

 have ground up for these regions, as for the Northern 

 States, a rich and abundant alluvium, sufficient in itself for 

 the production of a rapid and vigorous vegetation. The 

 South has apparently only the sittings of our Northern soil, 

 carried down to the ocean by rivers, and then washed up by 

 the sea-waves to form their interminable sandy plains. But 

 to compensate for this natural infertility of soil, the atmos- 

 phere, especially of Southern Florida, abounds in all the 

 elements of plant growth. The winds which come up from 

 the Gulf on one side, or the Atlantic on the other, are 

 charged with moisture, and bear also minute quantities of 

 nitric acid and saline compounds ; while ihe exhalations 

 from the swamps and marshes furnish in abundance the 

 salts of ammonia and carbonic acid. Now to utilize these 

 precious products from the air, it is necessary for plants to 



♦Read before the Sub-section of Microscopy of the A. A. A. S. 



