Mr William Galbraith on the Tides and Dew- Point. 7 
tend, as I suppose, to become assimilated ; so, during a con- 
tinuous night of clear atmosphere and dry frosty air, the ordi- 
nary diversity of electricity, it may be imagined, would be not 
only restored but increased. The engagement of electricity 
in the phenomena of chemical operations, and in every change 
in the constitution of natural substances, or of combinations 
in physical elements, necessarily suggests a reference to this 
principle for an explanation of the erystallizations described ; 
whilst the ascertained fact of the existence of a prevalent 
diversity of electrical condition betwixt the air and the earth, 
especially during the night, renders such reference at least 
plausible, whilst considering a reason for the vertical position 
and elevation of these interesting crystallizations arising out 
of the ground. 
On the Tides and Dew-Point. By W1LLIAM GALBRAITH, 
M.A., F.R.A.S., Teacher of Mathematics. Communicated 
by the Author. 
I. On the Tides. 
The phenomena of the tides have been long known; and it was 
frequently inferred, by repeated observations, that they had some re- 
lation to the motions of the sun and moon, It was not, however, 
till the time of Newton that any approach was made to their true 
theory. 
The investigations of Euler, Bernoulli, and Maclaurin, contributed 
much to perfect Newton’s views. Of late years renewed attention 
has been paid to their phenomena by Laplace, Lubbock, Whewell, 
Airy, and Daussy. Improved methods of observation and registra- 
tion have been introduced, though these are still capable of greater 
perfection. Tide-gauges of a superior construction have been 
erected at several important points, but not so numerous as could 
be desired. 
A very able marine surveyor has observed to me, that he could 
have wished it had come within my scope, in the new edition of 
Ainslie’s Surveying, edited by me, to recommend the general adop- 
tion of self-registering tide-gauges, where I gave the requisite for- 
mulw of reduction hitherto omitted in all works on the subject in this 
country. 
These tide-gauges, he observes, can be manufactured for about 
£30, complete, including a clock, and along with it such a register 
of the tides by night as well as by day. He remarks that Hewitson 
of Neweastle possesses a beautiful instrument of this kind; and my 
