Sept. 24, 1874] 



NATURE 



431 



part of Dr. Schmidt's and C'apt. I'upman's observations, ami has 

 incUided them with his own former collection, thus forming a 

 very extended catalogue founded on all the similar work of his 

 contemporaries and predecessors, and omitting but few genuine 

 meteoric showers, chiefly in the southern hemisphere, which have 

 only been observed by Dr. Neumayer in Australia. 



J-'olIowing the method of Dr. Weiss, viz. to calculate the radiant 

 points of those comets of early and recent times whose orbits are 

 believed to pass near the earth, a list of such comets for both the 

 northern and southern hemispheres is annexed to Mr. Greg's 

 catalogue, and the cases where they corroborate each other are 

 pointed out. Many important and well-known comets are found 

 to have meteor-showers as their present representatives, as 

 would, perhaps, be still more apparent if more reliable orbits of 

 comets could be used ; but the coincidences are, however, 

 numerous enough and sufliclently exact to render desirable the 

 further cultivation of cometary astronomy by the help of star- 

 shower observations. 



Report on Isomeric Crcso/s, by Dr. Armstrong. — Little has 

 been done by the committee during the pait jear. Fara s.\\i 

 orlho cresols have been obtained from ordinary cresylic acid, but 

 it has not been with certainty determined whether the tnota cresol 

 is likewise present, or whether these are the sole constituents of 

 this substance. 



Ki'port 0/ the Committee for the Ulilisation of Sa.uai;e, by Prof. 

 Corlield. — The committee has been unable, from want of funds, 

 to carry on the quantitative experiments as they would have 

 wished. Of the total nitrogen supplied to the farms during the 

 year March 25, 1S73, ^^ March 24, 1S74, 377 per cent, was 

 recovered in the crops, during the preceding year 4i'7 per cent, 

 was recovered, while during the first year of the experiments the 

 nitrogen recovered amounted to 26 per cent. Tlie committee 

 will be enabled, through the liberality of a gentleman, to carry 

 on their investigations during another year. 



SECTIONAL PROCEEDINGS 

 SECTION A— M.\THEM.\Tics 



On the Conslnution of a perfectly Achromatie Telescope, by Prof. 

 G. G. Stokes. 



At llie meeting of the Association in Edinburgh, in 1S71, 

 it was stated lliat it was in contemplation actually to con- 

 struct a telescope by means of discs of glass piepartd by the late 

 Mr. Vernon Ilarcourt, which should be achromatic as to secon- 

 dary as well as to primary dispersion. This intention was subse- 

 quently carried out ; and the telescope, which was constructed by 

 Mr. Howard Grubb, was now exhibited to the Section. The ori- 

 ginal intention was to constiuct the objective of a phosphatic glass 

 containing a suitable percentage of titanic acid, achromatised by 

 a glass of terborate of lead, ihe percentage of titanic acid was 

 so chosen that there should be no irrationality of dispersion 

 between tlie titanic glass and the terborate. As the curvature of 

 the convex lens would be rather severe if the whole convex 

 power were thrown into a single lens, it was intended to use two 

 lenses of this glass, one in front and one behind, with the con- 

 cave terborate of lead placed between them. It was lound that 

 provided not more than about one-third of the convex power 

 were thrown behind, the adjacent surfaces might be made to tit, 

 consistently with the condition of destroying the spherical as well 

 as the chromatic abeiration. This would render it possible to 

 cement the glasses, and thereby protect the terborate, which was 

 rather liable to tarnish. At the time of Mr. Ilarcourt's death 

 two discs of the titanic glass had been prepared, which it was 

 hoped would be good enougli lor employment, as also two discs 

 of terborate. These were placed in Mr. Grubb's hands. On 

 polishing, one of the titanic discs was found to be too badly 

 striated to be employed ; the other was pretty fair. As it would 

 have required a lather severe curvature of the first surface and 

 an unusual convexity of the last to tlirow the whole convex 

 power into the first lens, using a mere shell of crown glass behind 

 to protect the teiborate. Prof Stokes thought it more prudent to 

 throw about one-sixth of the whole convex power into the third 

 or crown-glass lens, though at the sacrilice of an absolute de- 

 struction of secondary dispersion, which by this change from the 

 original design might be expected to be just barely perceptible. 

 ( )f the terborate discs, the least striated happened to be slightly 

 muddy from some accident in the preparation ; but as this signi- 

 fied less than the striie, Mr. (Irubb deemed it belter to employ 

 t his disc. The telescope exhibited to the meeting was of about 



2.^ in. aperture, and 2S in. focal length, and was provided 

 with an objective of the ordinary kind, by which the other could 

 be replaced, for contrasting the performance. "When the tele- 

 scope was turned on to a chimney seen against the sky and half 

 the object-glass covered, in the case of the ordinary objective, 

 vivid green and purple were seen about the two edges, whereas 

 with the Ilarcourt objective there was barely any perceptible 

 colour. It was not, of course, to be expected that the performance 

 of the telescope should be good, on account of the difficulty of 

 preparing glass free from stria;, but it proved to be quite 

 sufficient to show the possibility of destroying the secondary 

 colour, which was the object of the construction. 



On Cyclone and Rainfall reriodicity in connection -with the 

 Sunspot Periodicity, by Charles Meldrum. 



The catalogue of cyclones experienced in the Indian Ocean, 

 from 1S47 to 1S73, submitted last year, indicated that during this 

 period the number of cyclones in the space between the equator 

 and 34" S. lat. and the meridians of 40' E. and iio° E. are 

 much greater in the years of maximum than in the years of 

 minimum sunspot frequency. 



It wiU now, and in subsequent reports, be shown that not only 

 the number of cyclones, but their duration, extent, and energy, 

 were also much greater in the former than in the latter years, and 

 that there is a strong probability that this cyclonic fiuctuation has 

 been coincident with a similar fluctuation of the rainfall over the 

 globe generally. 



The present commiuiication is confined to the twelve years 

 1856-67, comprising a complete sunspot cycle. 



With regard to tlie c) clones of the Indian Ocean, the investi- 

 gation is based upon the extensive collection of obicrvations made 

 by the Meteorological Society of Mauridus on the assumption 

 that the observations are so numerous that no cyclone of any 

 considerable extent or violence can have escaped detection. 



A chart has been prepared for noon on each day of the period 

 during which a cyclone lasted. The chart shows the positions 

 of the vessels, the directions and force of the wind, the state of 

 the weather and sea, &c. In this way the position of the centre 

 of the cyclone is ascertained for each day ; then, by examining 

 the several charts, the duration, extent, &c. of the cyclone arc 

 determined. 



The number of cyclones thus examined for the twelve years is 

 113, and their tracks have been laid down on six charts. 



1 he total cyclonic area in 1S60 and lS6i was about twelve 

 times greater than in 1S56 and 1S57, and neaily eight timis 

 greater than in 1S67 ; in sliort, all the factors were greater in the 

 years of maximum sunspot frequency. It is evident from the 

 table that the cyclonic area increased rapidly from 185S to 1S60, 

 and diminished slowly from 1861 to 1866. The registers for ilie 

 years 1856, 1857, 1866, and 1867 have been examined with 

 special care in oider that nothing might be omitted ; and, to gi\e 

 the utmost possible weight to those years, every instance of even 

 an ordinary gale has been taken into account. In 1S56 theie 

 was no great hurricane at all, and the same may be said of 1S57, 

 1S66, and 1867. From the chart for 1S06 it will be seen ih^t 

 in April of that year there was a number of small cyclones. The 

 south-east trade-winds and north-west monsoon were in collision 

 for a considerable time, aud several cyclonic eddies of short 

 duration were formed. 



If we could obtain good values of the mass of air in motion 

 and tlie velocity of the wind, it would probably be found that 

 the ratios of cyclonic energy were greater than those of c; - 

 clr>nic area, for in the ntaxima yeais the cyclones were mu^ii 

 more violent than in the minima years. Assuming the mass to 

 be nearly proportional to the area, and ihe velocity of the wind 

 in a strong gale to be 55 miles, in a whole gale 70 miles, and 111 

 a hurricane 85 miles an hour, the amount of cyclonic energy in 

 1S60 was about eighteen times greater than in 1856, the squares 

 of the velocities being as three to five. 



Although the results are necessarily rough approximation >, 

 yet the fact that the number and violence of the cyclones of ycais 

 of maximum sunspot were far greater than in the years ol 

 minimum sunspot is beyond all doubt. 



When a great hurricane takes place in the Indian Ocean, the 

 disabled ships are obliged to put into the nearest port, and tlie 

 newspapers in their shipping intelligence announce the arrival 

 of the vessels, the dates and localities of the bad weather, and 

 the amount of damage sustained. For upwards of twenty years 

 the Coin menial Gazette oi Port Louis has published all arrivals 

 of vessels and all maritime events which have been reported by 

 them. Considering, then, the geographical position of Mauritius, 



