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neglected power. The model in the Exhibition is a rough 
one, and the power exerted small. The idea is, however, at 
this moment like what the embryo windmill steam-engine once 
was — well worth pondering over. It must, however, be con- 
stantly met with antagonistic considerations, and especially 
must its probable results be compared with the known effects 
realised by steam. Mr. Tommasi’s model presumes that a 
large air-tight reservoir could be built in which the water of 
the tide, flowing through a pipe and rising inside, would com- 
press the air within. This condensed air would be drawn off 
as required in the service of the engines. Now two considera- 
tions instantly present themselves. First, the tides rise and 
fall in very few places to the extent of twenty feet. Twenty 
feet head of water would give the compressed air only a 
pressure of 10 lbs. on the square inch. Steam works up to 
60 and 70 lbs. pressure, and, therefore, as a motor, would 
be six or seven times superior. But steam costs constantly 
money for the supply of fuel to keep it up ; the tides do their 
work for nothing. We have then the interest of the money 
cost of the reservoir to put against the cost of the coal burnt. 
A five-horse power engine would consume some sixty tons of 
coal in a year’s incessant work ; the cost of a tidal air-compressing 
reservoir for a five-horse power engine would be, it is said, 
800£. ; the interest of this at 5 per cent, would be 40 1. We 
might reckon the coal at 15s. per ton, or 4:51. One does not see 
the economy of air. This line of thought, however, should not 
be condemned. To utilise the tides would be a world-wide 
benefit ; but the idea has not yet been contemplated long 
enough or deeply enough for universal application. There is 
in all probability a way to win. 
Thomson’s road-steamer, with india-rubber tyres ; Hodgson’s 
wire tramway, with the saddles of the buckets clinging on to the 
wire-rope by simple adhesion ; Grirdwood’s copper- wire steam- 
packing, the condensation of water within which forms the 
lubricant; Siemen’s electrical pyrometer, for measuring the 
degrees of very high temperatures ; Michele’s cement-testing 
machine, in which the bent lever is most ingeniously applied; 
Captain Scott’s selenitic cement ; are other examples of really 
useful and practical inventions, each of which might well 
have a page to itself to do justice to it. Mr. Grallo way’s and 
Mr. Warsop’s aerated steam is a topic also well worthy of close 
investigation and consideration. 
Amongst instruments of precision and fine philosophical 
workmanship, very notable are Sir Joseph Whitworth’s instru- 
ment for measuring to the one-millionth part of an inch, and 
the astronomical apparatus of Messrs. Cooke, of York. 
In a short notice like the present one cannot deal in detail 
