250 



NA TURE 



[yttly 14, 1 88 1 



Mr. Crudgington, of the Baptist Congo Expedition, has 

 just returned to England for a short time, and reports that by 

 now Messrs. Comber and Bentley will have formed a first station 

 at Isangila, Mr. Stanley's second post, and will at once push on 

 to Mbu, some sixty or seventy miles further along the north bank 

 of the Congo, where the the second station will be erected. It 

 is for this navigable portion of the river that a steel boat is 

 required, so as to avoid the Basundi. In his late journey up the 

 Congo Mr. Crudgington found these people warlike and trouljle- 

 some, as Mr. Stanley had done, and they were a source of per- 

 petual anxiety to him. The practicability of utilising this part 

 of the river is shown by the fact that Mr. Crudgington and his 

 party went over it in heavy, clumsy, native canoes ; but from 

 Mbu the expedition will have to go to Stanley Pool by land, as 

 the rapids and falls render the river quite unnavigable. The 

 plans of the expedition are now on such an extended scale that 

 six additional missionaries will be required— two for San Salva- 

 dor, one for the depot at Mu-^suca on the Lower Congo, one 

 each for Isangila and Mbu, and four for Stanley Pool, so that 

 occasional journeys may be made higher up the Congo. The 

 steel boat required by the expedition has been presented by an 

 anonymous donor at a cost of aljout 400'. It has been built in 

 London from drawings furnished by Mr. H. M. Stanley. 



The Times last week published an exceedingly interesting 

 letter on trade and exploration on the Congo from a private cor- 

 respondent at the mouth of the Ogowe. Speaking of M. 

 Savorgnan de Brazza, the writer says that he has done much to 

 open up the country betiveen the Ogowe and the Congo, that he 

 purchased a large tract of country near the sources of the former 

 river at a very cheap rate, erected a station, and left a white 

 man in charge. He is said to have purchased villages as they 

 stood, freed a great many slaves, and engaged them at monthly 

 wages to cultivate the plantations and keep the ground in order. 

 He seems to have been regarded as the apostle of freedom in the 

 country ; troops of slaves came flocking to him to be freed, and 

 his visit is regarded as having struck a blow at slavery in West 

 Africa. The writer gives a very different picture of the state of 

 affairs on the Belgian road along the north bank of the Congo. 

 It may be interesting to mention, the observations respecting the 

 light in which M. de Brazza is viewed by the natives are fully 

 confirmed by a letter from a Roman Catholic missionary who 

 accompanied him up the Ogowe last December. 



WHIRLED ANEMOMETERS^ 

 TN the course of the year 1S72 Mr. R. H. Scott, F.R.S., 

 suggested to the Meteorological Committee the desirability 

 of carrying out a series of experiments on anemometers of dif- 

 ferent patterns. This suggestion was approved by the Com- 

 mittee, and in the course of the same year a grant was obtained 

 by Mr. Scott from the Government grant administered by the 

 Royal Society for the purpose of defraying the expenses of the 

 investigation. The experiments were not however carried out 

 by Mr. Scott himself, but were intrusted to Mr. Samuel Jeffery, 

 then Superintendent of the Kew Observatory, and Mr. G. M. 

 Whipple, then First Assistant, the present Superintendent. 



The results have never hitherto been published, and I was not 

 aware of their nature till on making a suggestion that an anemo- 

 meter of the Kew Standard pattern should be whirled in the 

 open air, with a view of trying that mode of determining its 

 proper factor, Mr. Scott informed me of what had already 

 been done, and wrote to Mr. Whipple, requesting him to place 

 in my hands the results of the most complete of the experiments, 

 namely, those carried on at the Crystal Palace, which I accord- 

 ingly obtained from him. The progress of the inquiry may be 

 githered from the following extract from Mr. Scott's report in 

 r-iturning the unexpended balance of the grant : — 



" The comparisons of the instruments tested were first insti- 

 tuted in the garden of the Kew Observatory. This locality was 

 found to afford an insufficient exposure. 



"A piece of ground was then rented and inclosed within the 

 Old Deer Park. The experiments here showed that there was a 

 considerable difference in the indications of anemometers of 

 different sizes, but it was not possible to obtain a sufficient range 

 of velocities to furnish a satisfactory comparison of the instru- 

 ments. Experiments were finally made with a rotating appara- 

 tus, a steam merry-go-round, at the Crystal Palace, which led to 



' " Discussion of the Results of some Experiments with Wliirled Anemc- 

 meters." Paper read at the Royal Society, May 12, by Prof. G- G. Stokes, 

 Sec. R.S. 



some results similar to those obtained by e-xposure in the Deer 

 Park._ 



' ' The subject has however been taken up so much more 

 thoroughly by Doctors Dohrandt and Thiesen (z'/ijV "Repertorium 

 fur Meteorologie," vols. iv. and v.), and by Dr. Robinson in 

 Dublin, that it seems unlikely that the balance would ever be 

 expended by me. I therefore return it with many thanks to the 

 Government Grant Committee. 



" The results obtained by me were hardly of sufficient value 

 to be communicated to the Society." 



On examining the records it seemed to me that they were well 

 deserving of publication, more especially as no other experiments 

 of the same kind have, so far as I know, been executed on an 

 anemometer of the Kew standard pattern. In i860 Mr. Glaisher 

 made experiments with an anemometer whirled round in the 

 open air at the end of a long horizontal pole,- but the anemo- 

 meter was of the pattern employed at the Royal Observatory, 

 with hemisjiheres of 375 inches diameter and arms of 5'72S 

 inches, measured from the axis to the centre of a cup, and so 

 was considerably smaller than the Kew pattern. The experi- 

 ments of Dr. Dohrandt and Dr. Robinson were made in a building, 

 which has the advantage of sheltering the anemometer from wind, 

 which is always more or less fitful, but the disadvantage of 

 creating an eddying vorticose movement in the whole mass of 

 air operated on ; whereas in the ordinary employment of the 

 anemometer the eddies it forms are carried away by the wind, 

 and the same is the case to a very great extent when an anemo- 

 meter is whirled in the open air in a gentle breeze. Thus, though 

 Dr. Robinson employed among others an anemometer of the 

 Kew pattern, his experiments and those of Mr. Jeffery are not 

 duplicates of each other, even independently of the fact that the 

 axis of the anemometer was vertical in Mr. Jeffery's and hori- 

 zontal in Dr. Robinson's experiments ; so that the greater com- 

 pleteness of the latter does not cause them to supersede the 

 former. 



In Jlr. Jeffery's experiments the anemometers operated on were 

 mounted a little beyond and above the outer edge of one of the 

 steam merry-go-rounds in the grounds of the Crystal Palace, sj 

 as to be as far as practicable out of the way of any vortex \\ hich 

 it might create. The distance of the axis of the anemometer 

 from the axis of the "merry " being known, and the number of 

 revolutions («) of the latter during an experiment counted, the 

 total space traversed by the anemometer was known. The 

 number (N) of (7//rt;'f«/ revolutions of the anemometer, that is, 

 the numljer of revolutions relatively to the merry, was recorded on 

 a dial attached to the anemometer, which was read at the 

 beginning and end of each experiment. As the machine would 

 only go round one way the cups had to be taken off and replaced 

 in a reverse position, in order to reverse the direction of revolu- 

 tion of the anemometer. The true number of revolutions of the 

 anemometer was, of course, N + «, or N - k, according as 

 the rotations of the anemometer and the machine were in the 

 same or opposite directions. 



The horizontal motion of the air over the whirling machine 

 during any experiment was determined from observations of a 

 dial anemometer with 3-inch cups on 8-inch arms, which was 

 fixed on a wooden stand in the same horizontal plane as that in 

 which the cups of the experimental instrument revolved, at a 

 distance estimated at abont 30 feet from the outside of the 

 whirling frame. The motion of the centres of the cups was 

 deduced from the readings of the dial of the fixed anemometer at 

 the beginning and end of each experiment, the motion of the air 

 being assumed as usual to be three times that of the cups. 



The experiments were naturally made on fairly calm days, 

 still the effect of the wind, though small, is not insensible. In 

 default of further information, we must take its velocity as equal 

 to the mean velocity during the experiment. 



Let V be the velocity of the anemometer, W that of the wind, 

 fl the angle between the direction of motion of the anemometer 

 and that of the wind. Then the velocity of the anemometer 

 relatively to the wind will be — 



VV--2VWcosfl -f W= («) 



The mean effect of the wind in a revolution of the merry will be 

 different according as we suppose the moment of inertia of the 

 anemometer very small or very great. 



If, as is practically the case, W be small as compared with V, 

 the correction to be added to V on account of the wind may be 



= " Greenwich Magnetical and Meteorological Observations," 1862, Intro- 

 duction, p. li. 



