8 Mr William Galbraith on the Tides and Dew- Point. 
friend, Mr John Adie, assures me he could furnish an excellent one 
at the same price, or even lower, though these not quite of a supe- 
rior quality. 
Our friends, Messrs Brysons, have already put up apparently a 
most complete one at Glasgow, giving a register of both tides and 
winds. From its position near the Broomielaw Bridge, I am afraid 
the register of the winds cannot be very accurate, by reason of the 
adjacent buildings ; which is the fault of the proprietors, not that of 
the constructors. I have been informed that they could furnish a 
pretty good and complete tide-gauge for about £25, or £9 less than 
what has hitherto been charged in England. 
I shall here, then, endeavour to supply the omission formerly al- 
luded to, by urging the propriety of erecting self-registering tide- 
gauges, and wind-gauges at all our sea-ports and lighthouses hitherto 
unfurnished with them, wherever there is free access to the flowing 
and ebbing of the tide, and the unbiassed direction of the wind.* 
It requires great caution and perseverance to record properly the 
rise and fall of the tide, as well as the precise time of high and low 
water, without these properly constructed gauges, though this may 
be sometimes indispensable in certain localities, or where they cannot 
be conveniently placed. If a calm bay be selected, to which there 
is free access of the tide, these may be recorded with tolerable ac- 
curacy in moderate weather. On rocky shores there are frequently 
narrow openings among the rocks, where a suitable position may be 
selected. The irregularities may be often obviated, or greatly modi- 
fied, by laying a quantity of sea-weed or wreck across the entrance, 
through which the tide must percolate; and a little care and inge- 
nuity will generally (except in storms) so regulate the flow as to en- 
able an observer to estimate both the time and the height with con- 
siderable accuracy. 
According to the theory of Newton and Laplace, the sun and 
moon, by their attraction, exercised upon the waters of the ocean, 
produce the tides which we observe. The tide may therefore either 
be produced by the sum of their attractions or by their difference, 
according to their relative positions in reference to one another and 
to the earth. The compound tide is very great towards the syzygies, 
that is, about new and full moon, for then it is the sum of the 
partial tides caused by the sun and moon respectively, and are com- 
monly called spring tides, while those at the quarters are called 
neap tides, and are the smallest, because they are caused by the ea- 
cess of the lunar tide above the solar. 
The spring tides are not all equally great, because the partial 
tides which concur to produce them vary with the declination of the 
* Without barometers and wind-gauges no accurate conclusions, in particu- 
lar cases, can be deduced from tide-gauges. 
