29 REPORT—1874, 
the outer limb clear and sharp. Captain Abney has, after many experiments, 
decided on using my method of development for the dry plates used at the transit 
of Venus, the formula being a saturated solution of carbonate of ammonia with 
sufficient pyrogallic acid, and bromide of potassium if required. 
Puysics. 
On Experiments at High Pressures. By Pref. T. Axnruws, F.RS, 
On the Teaching of Practical Physics, By Prof. W. F. Barrerr, 
= 
On the Physical Theory of Undercurrents, By W.B. Carpenter, M.D., F.RS, 
On the Flight of Birds. By Prof, F. Gurarin, F.R.S. 
Confirmation of the Nebular Origin of the Earth. 
By G. Jounstone Srovny, 7. 2.S. 
a 
On the Physical Units of Nature. By G. Jounstoyn Sronny, FS, 
Physics of the Internal Earth. By Dr. Vavenan. 
“Hear, 
On the Latent Heat of Gases. By J. Dewar, F.R.S.E. 
On a New Class of Hydrates. By Prof, F, Gurnee, RS, 
——— 
On the Source from which the Kinetic Energy is drawn that passes into Heat in 
the Movement of the Tides. By Joun Pursnr, M.A., M.R.LA., Professor 
of Mathematics in Queen’s College, Belfast. 
Attention has of late years been directed, by Mayer, James Thomson, and others, | 
to the fact that the friction of the tidal currents on the bed of the ocean exercises 
an effect in retarding the earth’s rotation on its axis. The late astronomer 
Delaunay showed that such effect was appreciable, and that it furnished a not 
improbable cause of the unexplained part of the secular inequality in the moon’s 
mean motion. 
He pointed out that inasmuch as the axis of the tidal spheroid is always behind 
the moon’s place, a couple is exerted by the forces of the moon’s attraction, which, 
on the one hand, retards the rotation of the earth, and on the other, by its reciprocal 
action, increases the dimensions of the lunar orbit. 
This alteration of the lunar orbit prevents us concluding, as we should other- 
wise do, that the kinetic energy which passes into heat in the movement of the 
tides has for its exact equivalent a corresponding quantity drawn from the store 
laid up in the earth’s rotation on its axis, 
