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magnetic measure, on account of the supreme importance of 
such a measurement in the theory of light. 
I may in conclusion refer to a possible objection. The force 
of ‘‘stiction”? may be supposed to interfere with the reading of 
the limiting position of equilibrium. Practically the objection 
is groundless, for we can always (force of stiction notwithstand- 
ing) attain this position very nearly. Having done so, a very 
slight tap on the base of the instrument is sufficient to free the 
disk and take it slightly out of focus, where it remains, Then 
move the plates forward by means of the micrometer screw until 
the guard plate again catches up the disk. We thus get the 
poate of equilibrium without the interference of stiction 
at all. 
Iam now having the instrument altered by Mr. Groves. A 
very light and flat gilt disk of mica suspended by silvered silk 
fibres will replace the aluminium disk, and the distance between 
the plates will be varied within very narrow limits, so as to show 
whether the cushion of air between the plates exercises any 
influence on the results, 
The range of tilting of the plates will also be increased so as 
to allow of the employment of a large number of cells, In this 
case the equations previously used must be replaced by equations 
of the forms— 
Sin (@+a)=2E%, 
Sina=#’E’, 
tan (@+a)—tana=c, 
where 4, #, and ¢ are known constants. In these equations we 
can, of course, take a very small, as before, and get a very 
approximate and easily obtained solution by using expansions to 
the third order of small quantities—as I shall show in a sub- 
sequent communication on the completion of my experimeuts. 
: . GrorGe M. MINCHIN 
Royal Indian Engineering College, Cooper’s Hill, 
December 1 
VELOCITIES IN TIDAL RIVERS 
PAPER “On the Relative Value of Tidal and Upland 
Waters in maintaining Rivers, &c.,” by Mr. Walter R, 
Browne, M. Inst.C.E., late Fellow of Trinity College, Cambridge, 
has been lately published by the Institution of Civil Engineers. 
‘The main object of it is to prove, as a general principle, though 
by no means applicable in every case, that the main agent which 
keeps clear the channels of tidal rivers is not the run of tide 
passing up and down them every twelve hours, but the upland 
or fresh waters which pass down them at the period of low 
water, more or less aided by the cozing out of salt water which 
has soaked into the banks while covered with the tide. 
The author, with a view to check his conclusions by actual 
experiment, resolved to investigate the actual velocity at the 
bottom of a tidal channel during an ebb tide; since it is clear 
that, whatever the velocity at the top, it is the bottom velocity 
alone which produces any scouring effect. In the very largest 
rivers, above the action of the tide, the bottom velocity differs but 
little from the surface velocity ; but in smaller streams it is gene- 
rally much less than the surface velocity, and the ratio between 
the two decreases rapidly as the depth increases. The case of a 
tidal river however is somewhat different, because then the level 
of the bottom is below the surface of the ocean outside, and this 
must havea certain effect in ponding back the river current. 
Accordingly two sets of experiments, made at a carefully-chosen 
spot on the River Avon at Bristol, showed that for about two- 
thirds of an ebb tide, and even when the surface velocity was at its 
highest, the bottom velocity was absolutely #z/. The water at 
the bottom then seemed to start suddenly into activity, and almost 
immediately assumed a velocity agreeing fairly with that observed 
in ordinary rivers above the tidal area. The two sets of experi- 
ments were made with different meters, at different states of the 
tides, and at different times of year; so that they amply con- 
firmed each other. The stillness of the bottom was further 
proved by the board, supporting the rod on which the meter was 
hung, coming up with a deposit of silt upon its surface, showing 
that, far from any scouring being in progress, actual deposition 
was taking place. The second set of experiments was the most 
accurate, the meter having been apenally made and tested for 
oa purpose, The results are plotted on the accompanying 
_ In the diagram the line AB represents the yrobable variation 
in the surface velocities as sketched from the various observa- 
tions, The small circles represent the observations made at 
different times by the meter, and the crosses represent observa- 
tions made, as a check upon these, by floats at the surface. It 
will be seen that the meter observations are by far the most satis- 
factory. The line CD is similarly sketched from the observations 
of the velocities at the middle of the depth. 
face, and just when the latter is beginning to fall off. The line 
EFG and the circles contiguous to it refer to the bottom velocity. 
It will be seen that it rises from nothing to a tolerably high 
value, with very great abruptness, just at the time when the sur- 
face velocity begins to diminish: it is probable, however, that 
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It will be observed . 
that here the maximum velocity is attained later than at the sur-- 
the change is not so much connected with this as with the de+ 
crease in depth, which is given by the line H K and the 
contiguous circles. These are plotted to one-tenth the scale of 
the velocity observations, and the sudden flattening of the curves 
about 12 noon marks the beginning of what the author terms 
the low water period. 
It is believed that the fact of a current having a high velocity’ 
at the surface and absolutely none at the bottom has not been 
previously observed, and it may have considerable bearing om 
the general theory of the motion of rivers, as well as on the more 
practical points dealt with in the paper. 
ON THE ECONOMICAL USE OF GAS-ENGINES 
FOR THE PRODUCTION OF ELECTRICITY* 
‘THE lecturer pointed out, that as long as the chief practical use 
of electricity was in telegraphy it was the quickness of action, 
rather than the ability to transmit large amounts of power to a 
distance, that formed the chief feature in the employment of 
electricity ; but that in this exhibition the numerous practical 
examples of the electric transmission of power, rather than the 
electric transmission of signals, formed without doubt the leading 
feature. 
Much had been heard about the dynamo-electric machines 
which generate the electric current ; but while electricians were 
* Abstract of a lecture delivered in Frenchin the Salle du Congres, at the 
Electrical Exhibition, Paris, by Prof. W. E. Ayrton, F.R S. 
