August I, 



1889] 



NA TURE 



319 



During 1888 there were eighteen instances of this, in which the 

 beginnings of the attendant magnetic perturbations, as shown by 

 the self-recording magnetograph, were so abrupt that it was 

 posible by this means to determine the time of the revoUition of 

 the sun, the average period thus found being twenty-six days and 

 eight hours. 



This behaviour of auroras and magnetic storms indicates that 

 any solar disturbance which may originate them has this power 

 during a limited portion only of its transit across the earthward 

 side of the sun. From April 1886 (o April 1889 inclusive, there 

 were in this country 188 such characteristic outbreaks of the 

 aurora. In twenty-six, observations were lacking ; but in the re- 

 maining 162, in every instance, bright faculse with or without 

 dark spots are known to have been located upon the sun's eastern 

 limb appearing by rotation. In those instances in which a 

 disturbance appearing by rotation failed to originate an aurora, 

 there was, as a rule, an increase in the number of stations report- 

 ing thunderstorms. Indeed, at such times there was always an 

 increase of thunderstorms, although the aurora when at its height 

 not unfrequently seemed to take their place, causing a temporary 

 decrease. From this it follows that the appearance upon the 

 sun by rotation of spots or faculae is a condition upon which the 

 appearance of the aurora or increase of thunderstorms to some 

 extent depends. M. A. Veeder. 



Lyons, New York, July 17. 



Do Animals Count ? 



Having studied Sir J. Lubbock's interesting book, I remem- 

 bered a fact observed by me, which, though it is not conclusive, 

 seems worth mentioning. I was amused some years ago to 

 observe the feeding of the young in a spavrow-house near 

 an upper window of my house. The old sparrow alighted 

 upon the smal! veran la of the sparrow-house with four living 

 cankerworms in his beak. Then the four young ones put out 

 their heads, with the customary noise, and were fed each 

 with a caterpillar. The sparrow went off, and returned after 

 a while again with four living cankerworms in his beak, which 

 were disposed of in the same manner. I was so interested and 

 pleased with the process that I watched it for some time and 

 during the following days. 



A fact which I have not seen noticed here in the extensive 

 sparrow literature, is that for a number of years sparrows begin to 

 build nests of dry grass and hay at the top of high treci. The 

 first I saw were large irregular balls placed on the tripod of 

 twigs. The entrance was on the inner side near the lower end 

 of the balls. Last year, I observed another form of the nests. 

 A strong rope formed of dry grass, as thick as a man's wrist and 

 as long as the forearm, is fastened only with the upper end 

 to strong branches at the lop of high trees. The rope's end 

 has a rather large ovoid shape with the entrance to the inside 

 near the end. Of such nests I saw last winter about a dozen 

 on the elms here in Main Street, near the College grounds, 

 and similar ones in i'utnam Avenue and other streets. A long 

 pole near my house strongly covered by a vine [Celastrtis scandens) 

 had such a nest for three years, used every year. 



In the sparrow-houses around my lodging the sparrows stay 

 throughout the winter ; commonly one male and three females 

 in every house, till in spring the superfluous females are turned 

 out. H. A. Hagen. 



Museum of Comparative Zoology, Cambridge, Mass., 

 July 15- 



The "Hatchery" of the Sun-fish. 



The fact that the "sun-fish" of the American lakes and 

 streams prepares a place for the deposit of its eggs and guards 

 them till hatched is widely known. Certainly is has long been 

 known and is recorded in all recent American works on fishes. 

 The first detailed statement of its nidification I know of was 

 published by Dr. John D. Godman, in his "Rambles of a 

 Naturalist," about 1830, and is reprinted with the third and 

 succeeding editions of his "American Natural History," 

 published in 1836, &c. 



Another quite full account of its nest-building and care of the 

 eggs was published, with an illustration, by the late Prof. L. 

 Agassiz in the Proceedings of the American Academy of Arts 

 and Sciences (vol. iii. pp. 329, 330, 1857). But the accounts of 

 Godman and Agassiz, as well as all others, and my own observa- 

 tions, fail to agree with those recorded in Nature (June 27, 



p. 202). The sun-fish, generally at least, simply clears a sub- 

 circular area whose diameter is usually about two or three times 

 its own length, and therein the female deposits her eggs. It 

 has generally been assumed that she alone or she and the male 

 in turn guards the nest. The idea, however, is only the result 

 of analogy from the observation of the higher vertebrates. It is 

 quite likely that the male fish is usually the guardian of the nest, 

 as in the case of the Gasterosteids, Cichlids, and Silurids. 



It should be added that the American sun-fish, although called 

 in some places roach and bream, is not at all related to the 

 English fishes so named, but is the representative of a family 

 {Centrarchidct) peculiar to and quite characteristic of North 

 America. This family is exemplified by about forty species, 

 referred to ten or eleven genera. The only species observed in 

 the Adirondack region is the Eiipomotis gibbosus, generally 

 known to European naturalists as Pomatis vulgaris or auritus. 

 The family is closely related to the Percids, and is indeed 

 considered to form a part of the latter by many naturalists, and 

 has nothing to do with the Cyprinids, to which the roach and 

 bream belong. Theo. Gill. 



Washington, July 17. 



Centrifugal Force and D'Alembert's Principle. 



I agree so cordially with the greater part of Prof. Minchin's 

 address to the Association for the Improvement of Geometrical 

 Teaching, delivered on January 19, 1889, and reported in 

 Nature of June 6, p. 126, that I feel the more induced to 

 enter a protest against his remarks on the subject of " centri- 

 fugal force." 



I admit that the name is not well chosen, and is often mis- 

 understood, but I contend that we want a name for certain forces 

 which are now called centrifugal, and which, until a better name 

 be suggested, we can do no better than to continue to call by 

 that name. 



If a train, passing round a curve at too great a rate, tears the 

 rails from the sleepers, we want a name for the force producing 

 this effect. When a train, running over a horizontal girder 

 bridge, produces a deflection greater than that due to its weight, 

 we want a name for the force producing this extra strain. 



The popular mistake is in regarding centrifugal force as a force 

 imparted to a body whose motion is being deflected, instead of 

 being imparted by such a body. 



When a wet mop is trundled, for example, the water does not 

 fly from it owing to centrifugal force, but owing to want of 

 sufficient centripetal force to keep it back. 



Prof. Minchin says : — " If we imagine a stone to be attached 

 to an elastic string, one end of which is tied to the hand, while 

 the stone is projected vertically upwards, the hand would ex- 

 perience an ^^/^zc/^zn/ pull. Are we thence to conclude that the 

 stone is continually acted on by an upivard force ? " 



From this illustration Prof. Minchin obviously objects to the 

 term "centrifugal force" as meaning a force imparted to the 

 body whose motion is being deflected. In this he is certainly 

 right, but this is no objection against its legitimate use as a force 

 imparted by such a body. 



Clerk Maxwell (" Matter and Motion," p. 97), says that " in 

 some popular treatises centripetal and centrifugal forces are de- 

 scribed as opposing and balancing each other. But they are 

 merely different aspects of the sane stress." Just so. But 

 because two classes of forces are different aspects of the same 

 stress, why, if sufficiently common and important, should they 

 not have distinctive names given them ? 



What I understand by centrifugal force is the reaction against 

 a force deflecting a body's motipn. 



Knowing the objections which have been raised to the use of 

 the term centrifugal as denoting such forces, I have endeavoured 

 to find some unobjectionable equivalent for it. For ottripetcil 

 force (which, strangely enough, is not generally objected to), I 

 have found v/hat seems to me a fair equivalent, viz. normal force, 

 defining force as normal when it produces deflection only, and 

 tangential yi\\tw it produces change of rate only, but I have never 

 been able to find a better name than centrifugal for the reaction 

 against normal forces. 



Prof. Minchin traces back what he considers to be the 

 " fallacy " of centrifugal force to D'Alembcn's principle, to which 

 he objects as "unnatural and unnecessary." I do not think ho 

 will get many to agree with him in this view. The hypothetical 

 reversal of the resultant forces in D'Alembert's principle maybe 

 unnecessary in the sense that we can do without it ; but as it 



