Sept, 2, 1875] 



JWA'IURE 



373 



ment of the rate of wave progress. His apparatus consisted of 

 three deep troughs, two circular and one rectangular, and the 

 steadiness of the motion in each was remarkable ; he compared 

 the velocities of the waves with the times of vibrations of 

 pendulums, and verified that in different sized troughs the rate 

 varied mversely as the square root of the diameter. The 

 experiments excited a good deal of interest. 



The Rev. S. J. Perry, of Stonyhurst College (one of 

 the members of the expedition to Kerguelen to observe 

 the Transit of Venus), read a paper on that event. Father 

 Perry illustrated his remarks by diagrams of the sun and the 

 planet, as seen from various stations, and gave a very interesting 

 explanation which was attentively listened to. Ke said that 

 although much prominence had not been given to the idea, he 

 believed that a very important reason why so much expense was 

 gone into in the expedition was that the distance of the earth 

 from the sun entered into the calculation of lunar tables. The 

 observations were not of any striking nature ; they were simply 

 to watch a black spot pass across the sun. There was nothing 

 exciting about it, except that when the observations had to be 

 taken they had to be very careful about the precise time, and 

 they had to observe the spot during the whole time of its passage. 

 Having pointed out with reference to his diagrams the reason why 

 the different stations were chosen, he denied the assertions that 

 had been made that Sir George Airy neglected Halley's method 

 of observation for Delile's ; the truth was he had rightly decided 

 in favour of Delile, but he had not neglected Halley. With 

 regard to the (Halleyan) stations in the extreme north, they were 

 left to the care of the Russians, and the English, French, 

 Americans, Germans and others occupied in the southern hemi- 

 sphere. As it was mid-winter, the sun was very nearly on the 

 line of the southern tropics and nearly vertical at ingress over the 

 eastern border of Australia. There were primarily five English 

 Government expeditions, but as these were subdivided, there 

 were, including private observers and those of India and the 

 Colonies, about twenty English stations of observation. His 

 station was Kerguelen, to the south-west of Australia, and after 

 arriving there they found that the Americans had taken the 

 station recommended by the members of the Challenger Expedi- 

 tion, but in spite of that they had plenty of time to look about 

 the island (which was a very barren place, about ninety miles 

 by forty-five), and they were fortunate enough to get a much 

 better position than the Americans, after all, by going a little to 

 the south-west. They had been told before they went out that 

 there was always a mist over the island, but, though that might be 

 the case in the north of the island, which had been chiefly visited 

 before, it did not apply to the south while they were there, and 

 they had not more mist than there would have been in London. 

 On the morning of the transit, which they expected to begin at 

 6-30, they rose at four, and at once made preparations for the 

 day's work. They were divided into three parties, and were so 

 placed that, with the Americans, they formed four parties, about 

 eight miles distant from each other. They saw the sun very well 

 until after six o'clock, at the first (his own) station, until almost 

 the time that Venus was coming on to the sun's disc, and they 

 had the external contact as well as could be expected, for there 

 never could be absolute certainty with regard to such a point. 

 They continued very well until they had taken the bisection by 

 the planet of the sun's disc, but then there was just one little 

 cloud that came and placed itself right over the planet and 

 remained till ten minutes after the commencement of the transit. 

 At the other stations they were able to make obsei-vations of the 

 ingress. At his station they were able to get observations of the 

 internal and external contact at egress, and a few photographs. 

 Father Perry added particulars of the result of observations at 

 the other stations as far as could be ascertained, and narrated 

 his experience of a cyclone in the Indian Ocean on the home- 

 ward passage. He added that during their stay on the island 

 they not only made astronomical observations, but also a series 

 of magnetic and meteorological observations; and the Rev. A, 

 E. Eaton was sent by .the Royal Society to study the botany of 

 the island. 



In answer to a gentleman. Father Perry said if they got the 

 results of the observations in seven years' time they would be 

 very lucky, as they had first to determine their longitude, and that 

 occupied a very long time. 



Prof. Osborne Reynolds read a paper On the Refraction of 

 Sound by the Atmosfhcre, in which he remarked that in previous 

 papers he had pointed out that the upward diminution of 

 temperature in the atmosphere (known to exist under certain 



circumstances by Mr. Glaisher's balloon a cents) must refract 

 and give an upward direction to the rays of sound which would 

 otherwise proceed horizontally, and it was suggested that this 

 might be the cause of the observed difference of the distinctness 

 with which similar sounds were heard on different occasions, 

 particularly of the very marked advantage that the night has 

 over the day in this respect. On this subject he had made a 

 series of experiments. He mentioned a case in which at sea, 

 when leaving a yacht in a small boat, for the purpese of makinir 

 experiments on sound, those in the yacht and the boat were able 

 to call to one another, and he heard at a distance of three-and- 

 a-half miles, and that the hiss and report of a rocket sent up 

 from the yacht was heard at a distance of five miles. Also on 

 the same occasion the barking of a dog on shore, which was 

 eight miles distant, was heard, and the paddles of a steamer 

 which must have been fifteen miles off were distinctly audible. 

 Prof. Reynolds remarked that the distinctness with Yvhich sounds 

 of such comparatively low intensity could be heard was perhaps 

 beyond anything definitely on record, although remarkable 

 instances of sounds heard a long way off were occasionally 

 heard •f. As the result of a series of experiments' made by 

 means of an electric bell. Prof. Reynolds found that when the 

 sky was cloudy and there was no dew, the sound could invari- 

 ably be heard much further with than against the wind ; but 

 when the sky was clear, and there was a heavy dew, the sound 

 could be heard as far against a light wind as with it. On one 

 occasion in which the wind was very light and the thermometer 

 showed 39° at one foot above the grass, and 47" at eight feet, 

 the sound was heard 440 yards against the wind and only 270 

 yards with it. 



The paper by Prof. G. G. Stokes and Dr. J. Hopkinson, On 

 the Optical Properties of a Tiiano-silicic Glass, we give in extenso 

 on account of its importance. At the meeting of the Associa- 

 tion at Edinburgh in 1871, Prof. Stokes gave a preliminary 

 account of a long series of experiments in which the late Mr. 

 Vernon Harcourt had been engaged, on the optical properties of 

 glasses of a great variety of compositions, and in which since 

 1862 Prof. Stokes had co-operated with him.* One object of 

 the research was to obtain, if possible, two glasses which should 

 achromatize each other without leaving a secondary spectrum, or 

 a glass which should form with two others a triple combination ; 

 an objective composed of which should be free from defects of 

 irrationality without requiring undue curvature in the individual 

 lenses. Among phosphatic glasses, the series in which Mr. 

 Harcourt's experiments were for the most part carried on, the 

 best solution of this problem was offered by glasses in which a 

 portion of the phosphoric was replaced by titanic acid. It was 

 found, in fact, that the substitution of titanic for phosphoric acid, 

 while raising, it is true, the dispersive power, at the same time 

 produces a separation of the colours at the blue, as compared 

 with those at the red end of the spectrum, which ordinarily 

 belongs only to glasses of a much higher dispersive power. A 

 telescope made of discs of glass prepared by Mr. Harcourt, was, 

 after his death, constructed for Mrs. Harcourt by Mr. Howard 

 Grubb, and was exhibited to the Mathematical Section of the 

 late meeting in Belfast ; this telescope, which is briefly described 

 in the Report, + was found fully to answer the expectations that 

 had been formed of it as to destruction of secondary dispersion. 



Several considerations seemed to m.ake it probable that the 

 substitution of titanic acid for a portion of the silica, in an 

 ordinary crown glass, would have an effect similar to that which 

 had been observed in the phosphatic series of glasses. Phosphatic 

 glasses are too soft for convenient employment in optical instru- 

 ments, but should titano-silicic glasses prove to be to silicic what 

 titano-phosphatic glasses have been found to be to phosphatic, 

 it would be possible, without encountering any extravagant 

 curvatures, to construct perfectly accurate combinations out of 

 glasses having the hardness and permanence of silicic glasses ; 

 in fact, the chief obstacle at present existing to the perfection of 

 the achromatic telescope would be removed, though naturally 

 not without some increase to the cost of the instrument. But 

 it would be beyond the researches of the laboratory to work 

 with silicic glasses on such a scale as to obtain them free from 

 striae, or even suficiently free to permit of a trustworthy deter- 

 mination of such a delicate matter as the irrationahty of dispersion. 



When the subject was brought to the notice of Mr. Hopkinson, 

 he warmly entered into the investigation, and thanks to the 

 liberality with which the means of conducting the experiments 



• Report for 1871. Transactions of the Sections, p. 38. 

 t Ditto for 1874. Transactions of the Ssctions, p. 26. 



