June 1, 1891.] 



KNOWLEDGE 



107 



to regions where the pressure is less, until, on reaching the 

 surface, it runs out as lava, and allows its occluded steam 

 freely to escape. 



ASTRONOMY AS TAUGHT BY ACADEMY 

 PICTURES. 



M 



. EYUE C'llOWE'S picture in tliis year's lioyal 

 Academy I'jxhibition, representing Jeremiah 

 Horroeks arriving in haste from his clerical 

 duties to find Venus on the Sun, is one of those 

 useful pictures which draw attention to a great 

 mail who has not been sufficiently known and appreciated, 

 and I therefore criticize it in no carping spirit. The picture 

 has already caught the public taste, and reproductions of it 

 have appeared in several illustrated papers. I am no ad- 

 vocate of too close astronomical criticism of a picture 

 which is not intended to teach Astronomy, but in this case 

 Mr. Eyre Crowe evidently does intend to present an object 

 lesson in astronomy, and he should either have devoted more 

 time to Horroeks' own account of his observation, or he 

 should have called at the Astronomical Society and applied 

 to the obliging Librarian for advice as to whom to look to 

 for help. 



Mr. Eyre Crowe has drawn Venus far too small, and 

 the beam of light should not fill the eyepiece of the tele- 

 scope. There is no authority for the curious I'Juuatorial 

 mounting which he has drawn, or for the attachment of the 

 sun-screen to the instrument by a rod ; in fact, we know 

 that such a mounting could nut have been used. Horroeks 

 had been examining the Sun all day , and through his narrow 

 windows he could not have followed the Sun with such an 

 instrument for more than half an hour at most. He 

 would continually have been obliged tu move and readjust 

 the position of the I'olar axis of his stand, a loss of time 

 and opportunity which the ingenious Horroeks would 

 never have risked. It is evident that the telescope was 

 attached to the shutter, or was on a stand immediately 

 behind a bole in the shutter. In the IV/jh.v in Salf i'ina, 

 in which H(jrrocks gives an account of his observation, he 

 says : " When the time of observation approached, I 

 retired to my apartment, and having closed the windows 

 against the Hght, I directed my telescope — previously ad- 

 justed to a focus — through the aperture towards the Sun, 

 and received his rays at right angles upon the paper already 

 mentioned." 



This paper bad upon it a six inch circle, carefully divided 

 into degrees, and a divided scale, which enabled him to 

 measure the diameter of the body of Venus, and the distance 

 of the planet from the Sun's limb. Horroeks goes on : 



" The Sun's image exactly filled the circle, and I watched 

 carefully and unceasingly for any dark body which might 

 enter on the disc of light. . . . Anxiously intent, therefore, 

 on the undertaking, through the greater part of the 2ard, 

 and the whole of the 21th, I omitted no available oppor- 

 tunity of observing her ingress. I watched carefully on 

 the 21th from sunrise to nine o'clock, and from a little 

 before ten until noon, and at one in the afternoon, being 

 called away in the intervals by business of the highest 

 importance, which, for these ornamental pursuits, I could 

 not with propriety neglect." 



The place of \'enus upon the Sun's disc is not ipiite low 

 enough on the right-hand limb in Mr. Crowe's picture. 

 Horroeks says that it was about " ()2° HO'." certainly 

 between (iO°and (i") ' from the top of the' Sim's disc towards 

 the right. The black disc reinesenting \'enus should be 

 larger; Horroeks estimated it to be about a (piarter of an 

 inch in dianu'ter or a little more, for lie thout;ht that 



Venus was more than 1' 12" and less than 1' 30 ' in 

 diameter. The size of the telescope drawn by Mr. Crowe 

 seems to be about right. The tube was evidently home- 

 made. Horroeks says that he gave 2s. 6d. for the object 

 glass. It is evident fi'om the description of his observa- 

 tion, thit Horroeks used what was then known as the 

 (ialilean form of telescope, that is, it had a negative and 

 not a positive eyepiece, so that Mr. Crowe is right in 

 drawing the emergent rays of light as not crossing after 

 they issued from the eyepiece ; but they should issue in a 

 narrower cone ; about a third of an inch in diameter near 

 to the eyepiece. One cannot speak more accurately without 

 Imowing the negative focus of the lens used for an eyepiece. 



Mr. Crowe should, I think, have drawn Horroeks in a 

 full cassock, as he is returning from service ; at this date 

 the clergy wore them at all times. The face seems to me 

 to be rather old for a man who could not have been more 

 than 22. As to the age of Horroeks, see a paper in the 

 Astronnwiral lici/istcr for December, 187-1. 



It is e\'ident that Horroeks could not have entered the 

 room and found the Sun upon his screen and Venus on its 

 disc, for he had no clockwork to drive his telescope. He 

 must have entered the darkened room, found the Sun, 

 and carried his screen to the distance where the Sun's 

 image would fill his divided circle. The picture would be 

 more accurate as depicting Horroeks' friend Crahtree than 

 Horroeks himself. For Crahtree did not measiu'e the 

 diameter of Venus and its place upon the Sun's disc, a 

 task to which Horroeks, in spite of his excitement, set 

 himself at once, and succeeded admirably. All interested in 

 spreading an interest in Astronomy must, however, be 

 thankful to Mr. Crowe, and I hope that he will have 

 many imitators who wiU dare to run the gauntlet of 

 criticism. A. C. Ranyaiu). 



THE TRAVELS AND LIFE-HISTORY OF A 

 FUNGUS. 



By J. Pentland Smith, M.A., B.Sc, kc, Lecturer on 

 Botany, Horticultural College, Swanley. 



WERE one to put the question " What is a 

 fungus?" to a non-botanical oliserver, he 

 would probably receive for an answer " A 

 nnishroom is a fungus," getting an example 

 of the group instead of a definition. 

 A mushroom is a fungus, and a readily observed member 

 of the group ; but the botanical tyro (piickly learns that 

 this plant cannot afford Imn a typical example of the 

 life-history of fungi, and that all do not make themselves so 

 evident to the sight as this pai'ticular fungus ; in fact, he 

 would soon perceive that the air is loaded with the spores 

 or reproductive cells of fungi which are ready to germinate 

 whenever they find suitable quarters. 



Within late years our knowledge of the lower members 

 of the vegetable kingdom has made enormous strides, and 

 results have been obtained by careful and prolonged study 

 which would astound the botanists of fifty years ago were 

 they now to appear upon the scene. Then it was believed 

 that Fungi and Bacteria were produced by the decay of 

 other living organisms, and that they were a lusus mitiinr, a 

 freak of Nature. Their sudden appearance on a spot 

 where no previous indication of their presence had been 

 manifested was adduced as an argument in favour of the 

 doctrine of spontaneous generation, or iiiinridio injuironi, 

 that from unorganized matter living beings may arise. 

 In later times Prof. Tyndall was an npliolder of this 

 theorv ; but as the outcome of his numerous and carefully 



