NOTES ON: TIDE-GAUGES. 79 
should be necessary to use a material so perishable as 
rubber, for when the bag is replaced with a new one, some 
difficulty occurs in bringing the pencil to the correct zero, 
and this of course is an essential to the adoption of the 
machine. Then it is obvious that, unless the air is pumped 
into the bag at a temperature equal to the sea water, and 
that this temperature remains practically the same during 
the time the machine is in operation, condensation will take 
place in the small copper tube, and the moisture, accumul- 
ating in the bends of the tube, will interfere with, if not 
wholly stop, the free transference of air pressure, and thus 
stop the working of the machine. Then again it is doubtful 
if any material, which could be used in the construction of 
the air drum C, has yet been discovered that would respond 
periectly to alternate expansion and contraction for an 
indefinite period. These objections go to show that for 
permanent installations the Richard tide-gauge is not as 
suitable as some others, notably the Kelvin machine, (Plate 
1), but for use on hydrographic surveys, where a gauge is 
seldom kept more than a few months in one place, it is 
invaluable. Hxtreme accuracy in these case is not required, 
so that the differences due to changes of temperature may 
be neglected, while any accumulation of moisture in the 
tube would be quickly noticed as the gauge is referred to 
nearly every day. 
Some years ago the author designed a method of doing 
without the flexible band, and connecting the float and 
the indicator by means of lazy-tongs (Plate 2). This 
idea was first suggested by Mr. H. Bale of the Public 
Works Department. Great care must be exercised in 
making the 19 joints of the lazy-tongs, as the pencil must 
respond at once to the least movement of the float, and 
this will not take place if there is any appreciable elasticity 
in the rods or any lateral play inthe pivots. The arrange- 
