SCIENTIFIC SUMMAEY. 267 
will be carried beneath the water-surface, on account of the formation of 
cavities, accordmg to the theory of Magnus. 
3rd. If there is not a great depth of water in the vessel which supplies 
the descending stream, or if (the depth not of necessity being small) rotatory 
motion is from any cause imparted to the water, air will enter through a 
cavity formed above the orifice from which the descendmg stream issues, and 
extending into the descending stream. 
4th. If the area of the cross section of the orifice from ■which the water 
flows be as great, or nearly as great as that of the tube through which the 
water falls, and if, a,t the same time, the orifices for the admission of air do 
not exceed a certain area compared with that of the orifice from which the 
water flows, air wiU enter at the rupture of the stream produced at the 
orifices by the accelerated motion of the water below' those orifices. 
An Improved Form of Colour-top. — Mr. E. Swfift, jun., of Liverpool, has 
transmitted to us the follow^ing description of a novel variety of this philo- 
sophical toy : — 
“ A very interesting optical instrument may easily be made by attaching 
to the colour-top known as Gorham’s a simple arrangement of frictional 
wheels, to give to the disks a velocity -with ease of from two to three thousand 
revolutions ui a minute. 
“ In the one I have had in use for the last three years the two large wheels 
are 12 inches, the second wheel 4 inches, and the small w'heel to which the 
spindle is fastened 1 inch in diameter. The coloured disks are 10 inches in 
diameter. 
“ Fastened to the box containing the wheels is a gas-pipe with two burners 
at each end, and a small branch pipe to convey the gas to the instrument by 
means of a flexible tube attached to it. The lights must be placed on exactly 
opposite sides of the disks. Shades made of tin or cardboard must be 
placed over the lights, which, though allowing the brightest illumination 
of the coloured disks, will entnely screen the light from the upper black 
disks. 
“ This arrangement gives a very great increase of brilliancy and purity to 
the colours by artificial light, and if made on a large scale and illuminated 
with the oxy-hydrogen or electric light, would form a beautiful uistrument 
for public exhibition.” 
ZOOLOGY AND COMPAKATIVE ANATOMY. 
Museums and their Uses . — The president of the zoological section of the 
British Association, Dr. J. E. Gray, diverged from the course usually adopted 
in delivering his openhig address to the Bath meeting. Instead of simply 
giving a compiled and sketchy account of the progress of zoology during the 
previous year, he adopted another and a far more useful and practical plan ; 
he read the section a lecture upon the subject of museums and their uses. 
This address he has since pubhshed in pamphlet form, and on that account 
it comes before us for special notice. The wuiter is entirely o^^posed to the 
present system of arrangement pursued in natural history museums, w'hich 
he does not scruple to observe are m many cases little better than huge 
