October 1, 1892.] 



KNOWLEDGE 



189 



researches bad been uninterrupted," as an eminent man of 

 science admitted.* Tbe whole paper should be studied. 



When the Woodwardian Professorship of Geology at 

 Cambridge became vacant in 1762 it was rightly felt that 

 Michell was the man for the post, " and there is every 

 reason to believe that had he retained the office for any 

 length of time he would have done much to rescue the 

 title of Woodwardian Professor, as well as the museum, 

 from the species of contempt in which both were long 

 allowed to remain. Unfortunately, however, after two short 

 years he took leave of Cambridge and the Woodwardian 

 possessions, attracted by the superior chai-ms of a wife and 

 a living. "+ 



Of these two crowning blessings the wife came first, the 

 li\-ing not until the end of another three years ; for after 

 176-4 Michell is not heard of at Queen's, and the natural 

 inference would be that he vacated his fellowship in order 

 to marry. It was obviously no flighty marriage, since he 

 was pretty nearly forty years of age at the time and his 

 wife seven years younger. Fortunately, marriage and his 

 living did not withdraw him from scientific work, for the 

 period between 1764 and 1767 (the latter year dating the 

 commencement of his residence at ThomhiU) was one of 

 the most fruitful periods of his life. 



In 1765 he was one of a committee of six (" three 

 gentlemen skilled in mechanics and three watchmakers '), 

 appointed by the Commissioners of Longitude, to examine 

 the chronometers then recently invented by .John Harrison. 

 An Act of Parliament had been passed in 1711, offering 

 for an exact chronometer a reward of " i'lO.OOO if it deter- 

 mine the longitude to one degree of a great circle, or sixty 

 geogi-aphical miles ; i'lo,000 if it determine the same to 

 two-thirds that distance : and £20,000 if it determine it to 

 one-half that distance "' : adding, " that one-half of such 

 reward shall be due and paid when the said Commissioners, 

 or the major part of them, do agree that any such method 

 extends to the security of ships within eightj- geographical 

 mOes from the shore, which are places of the greatest danger ; 

 and the other half when a ship by the appointment of the 

 said Commissioners, or the major part of them, shall 

 sail over the ocean from Great Britain to any such port in 

 the West Indies as those Commissioners, or the major part 

 of them, shall choose or nominatefor the experiment, with- 

 out losing her longitude for the limits above mentioned." 

 After many delays and much bickering from Harrison, who, 

 asapracticalman, had strong objections to "men of theory," 

 the committee was made up of " the Rev. .John MicheU, 

 late Woodward Professor of Geology, the Rev. Will Ludlam, 

 Fellow of St. -John's College, Camb.. John Bird, mathe- 

 matical instrument maker, with Thomas kludge, William 

 Matthews, and Larcum Kendal."^ 



A better man than Michell eotild not have been selected 

 for this examination, and it is pretty clear that the 

 committee as a whole did then- duty very well. Harrison's 

 chronometer was found on trial to keep the time so 

 accm-ately as to allow the longitude to be ascertained to 

 within one quarter of a degree, a perfection not anticipated 



* Sir Chiirles Lvell : J'riiiciples of Geologti, 9th ed., p. 41. 



t From an historical sketch of the Woodwardian lluseiun in 

 Smith's Cambridge Portfolio, bv David Ansted, formerly Fellow of 

 .Tesus College. Sir David Brewster has observed that Michell held 

 the Woodwardian Professorship for eight years, and Lyell repeats the 

 statement. But ilichell eoidd not have held it "for more than two 

 years, for in 176.5 he is spoken of as laie Woo<lwardian Professor; 

 nor would he have been allowed to hold that office and live at 

 Thomhill at the same time. It is said that we owe to Michell manv 

 of the terms now in use among geologists, such as " Gault." '* Coal- 

 measures, " &c. See also Woodward's Geology of England and 

 Wales. 



X Dodsley's Annual Segi»ter for 176.5. 



by Parliament. The clever but slightly over-reaching 

 watchmaker at once thought himself entitled to the prize, 

 without disclosing the principle of his instrument, a con- 

 sideration which would have completely nulhfied the 

 purpose of the Act. Then the bickerings commenced 

 afresh— Michell, and those with him, rightly declining lo 

 reccmmend the award unless the whole plan and construc- 

 tion of the chronometer were forthcoming ; Harrison, 

 whose temper was none the more docile for certain shabby 

 treatment he had received from the Government, declining 

 to divulge it, claiming that he had done all that the law 

 required in makiinj the instrument — and it was not imtil 

 many words had been wasted on all hands that he could 

 be persuaded to give way, to the great profit of 

 navigation.; 



Michell next appeared before the scientific public with 

 two useful papers on the use of Hadley's quadrant for 

 surveying and pilotage, and on a new method for measur- 

 ing degrees of longitude, respectively, neither of which, 

 however, need be explained here. 



Finally, before settling down as a country parson, he 

 published " An Inquiry into the probable Parallax and 

 Magnitude of the Fixed Stars, from the quantity of light 

 which they afi'ord us, and the particular ckeumstances 

 of their situation."* A finer paper has probably never 

 been read before a scientific assembly — indeed, modem 

 stellar astronomy has not got so very far beyond the veri- 

 fication of the conclusions it set forth. First, in a course 

 of elegant arguments, Michell shows that the nearest fixed 

 star is not very likely to be more remote than 220,000 

 times the radius of the earth's orbit. His procedure is 

 simple, so simple indeed as to make one wonder why it 

 was not thought of before. He compares the total light 

 received from the sun with the average amount received 

 from the first magnitude stars. To do this with absolute 

 accuracy it would be necessary in the first place to obtain 

 the actual amount of light received from the brightest 

 stars by some such method as the use of, say. Prof. 

 Pickering's photometer, or a wedge, and then to compare 

 this with the light of the sun. But this would be a 

 most laborious process, and not to be lightly undertaken ; 

 nor, in truth, were any such accurate means of light- 

 measurement then known. Yet, even though the days of 

 wedge and meridian photometers had not come, Michell 

 could still show a very short and simple way of dealing 

 with the question. Consider the planet Saturn when in 

 opposition, and when both Saturn and the earth are at 

 their respective mean distances. In this case, it takes but 

 a rough observation to show that Saturn is not much 

 brighter than the average first magnitude star. Take 

 away the planet's ring and they will be nearly equal. 

 Now, assuming that Saturn reflects all the sun's light 

 which falls upon him — an assumption not strictly true, 

 but near enough — and combining this with the con- 

 sideration that the Ught received from a bright area varies 

 inversely as the square of its distance, we have the follow- 

 ing easy calculation : — The intrinsic brightness of Saturn 

 is to the intrinsic brightness of the sun as the square 

 of the distance of the sun's surface from its centre is to the 

 square of Satm-n's distance from that centre. Now, 

 Saturn is about 2082 times fiu'ther from the sun's centre 

 than the sun's sm-face is ; therefore the brightness of 

 Saturn is to the brightness of the sun as unity is to 

 2082 X 2082. Again, the sun's diameter is about 105 

 times the diameter of Saturn ; hence the sun's disc 

 will be 105 X 105 times greater than Saturn's, And to 



§ Ibid. 



II Fhil Trans, for 176.5 and 1766. 



T Phil. Trans., Vol. LVII., p. 234. 



