THE TRANSIT OF VENUS.

Evening Star, Issue 3712, 15 January 1875, Page 2

THE TRANSIT OF VENUS.

c _ , To tht -Editor. tWh7v. ,"P tb « P««*. in fsSrß-j-HP’s toons, and that », ve ” 8 observaof Mara and the Moon ° bßervatioM determining the velocity o7 the re-examination of the fw tions by Professor Stone said to lv> canal »„i . 1 , 8 difference is few. oK Sm»“S B',b- ***'“ rery

certain extent, valuable te a clearstatementof some of the conditions of the tran-it problem, is worthless as a criticism on the letters of “ Innuiier” and myeelf. Certa n date viz., a human hair and 8,000 feet wore given by “ Inquirer,” along with a certaiun suit, said to have been obtained fi«m them. It w-s easy enough to chow that the result given was erroneous. This was all that I undertook to do. Of course “ Pee>ybingle” may, if he wishes to do so, say that “Inquirer” and myself are “at sea” with regard to this matter, but I scarcely think the tacts of the case wiM warrant his making any such assertion.—l am, Ac., Rough Astronomical Notes. Dunedin, January It. P S It ought perhaps to be mentioned that " Peerybingle’s” version of Professor Peters’s statement is quite as open to abjection as the ene before referred to. It is not true that, astronomers expect to be able “to measure the thickness of a hair at a distance of 8,600 ft.” What they really expect is to be able to measure a certain angle, so that the error shall not exceed the angular distance of a hair at B,OOOft—which is a very different matter. If a carpenter were told to cut wood from a plank ia such a way that it might be within 1 16th of an inch of a certain length, it is plain that this would be very different from te I ling him to cut off exactly 1 16tb of an inch. It is true that a ca renter’s rul** may be said in a rough sort of way to be capable of measuring I-I6th of an inch, but as bp exact measure of such a quantity it would prove to be but a poor micrnmrter. Perhaps ‘ P j ervbin?le” will kindly let us know what is to be tbe “limit of error” in the measurement of this l-106th part of a second of bis If l-'Oth of the whole, then ast’-onornera will have a very fair chance of getting not onlv the second but the third decimal place. But we have not yet done What baa been measured is. in accordance w'th “ P< erybingle’s ” theory, not the one hundredth of an inch, but. the limit of error—the one-thousandth Again, what is the limit of error? Say 1 10th of the whole quantity. Then we shall be in a position to get the required measure correct to fiv- places, and so on ad infinitum. On the whole, if “ Peerybiogle” is not “at sea” bo is up to his neck in a quicksand. To the Editor. Sir. —Both y<mr correspondents who have replied to mv letter appear to have mi understood the nature of my question, and both seem to have arrived at the same conclusion : that Professor Peters has been impo'fectly heard or v accurately reported. N ow, sir, this is not the case. Professor Peters cer tainly did make the statement as reported, viz . “that the angle which they had to asc«>tain was equ«l to an angle formed bv a human hair extended at a distance of 8,000 feet,” giving Sir John Herschel as his authority. and repeated—“We have to measure the tbtekneas of a human hair at a distance of B.oo'lfeet.” What T wished to know was in what work of Sir John Herschel’s this information is to be found ? I am perfect'y well acquainted with all that your “ Hough Astronomical Noter” has communicated, yet I thank him most heartily for the trouble he has taken in adopting the method of conveying his information so clearly, and, paradoxical as it may appear, so concisely in a “roundabout” way. There is a considerable discrepancy bet > een the result obta’ned by him and that obtained by Sir John Herschel. This is owing probably to Sir John having in his calculation used a thicker hair say one of 1-390 th of an inch ia thickness and a larger angle, viz ( "35) thirty-five hundredths of a second, which is about the difference between tbe old value of the sun’s parallax as obtained bv Beisel from the transit of Venus in 1769. and the mean of those obtained by Hansen from th“ moon’s parallactic equation, Winne ke from the observations of Mars. Fou-ault from the velocity of light, I everrier from the motions of Mars and Venus and the apparent motions of the sun, and other observation and calculations by Stone and Ellery. The old value = 8.678 New value (mean of observations given above) = 8.924

Difference .346 second The distance is said by Herechel to be about 125 feet—by your correspondents 175 feet—and is given simply with the view of conveying an impression of the extreme minn'eness of the angle, which r- duces the distance of the sun from the earth by nearly four million of miles. To suppose for a moment that Professor Peters m de the statement through ignorance. o r is himself incapable of working out the problem, is simply absurd. No greater / astronomer than the learned Professor lives at tbe present time. Had he made tbe statement on his own account I should not have noticed it; but having given the statement.as Her ehel’s, if he has not been misunderstood, then it will rather add to than detract frono the dignity of his position in the scientific world if be acknowledges his error in this particular.—l am, &c.. Inquirer. P,S.—Since writing the above, I have seen the letter of your correspondent, “ Peerybing'e,” in last evening’s issue. His remarks do not bear upon tbe question I asked. Possib'y “ ' ouch Astronomical otes ” may wi hj to tread hut one measure with him If so, let him look ou for bis toes as they pags through tbe figures together.—l am fee . INQUIRER. Dunedin, January 14.