354 PHYSICAL LABORATORY OB' GREAT BRITAIN. 



The large gauge became unsafe in 1896 and was removed, but the 

 observations for the previous ten years entirely confirm this result, the 

 importance of which is obvious. The same result may be deduced 

 from the Tower Bridge observations. Power is required to raise the 

 great bascules, and the power needed depends on the direction of the 

 wind. From observations on the power some estimate of the average 

 wind pressure on the surface may be obtained, and this is found to be 

 less than the pressure registered by the small wind gauges. 



Nor is the result surprising when the matter is looked at as an hydro- 

 dynamical problem — the wind blows in gusts — the lines of flow near a 

 small obstacle will differ from those near a large one; the distribution 

 of pressure over the large area will not be uniform. Sir W. Siemens 

 is said to have found places of negative pressure near such an obstacle. 

 As Sir J. Wolfe Barry has pointed out, if the average of 50 pounds to 

 the square foot is excessive, then the cost and difficulty of erection of 

 large engineering works is being unnecessarily increased. Here is a 

 problem well worthy of Attention and about which but little is known. 

 The same, too, may be said about the second of the Board of Trade 

 rules. What is the effective surface over which the pressure is exerted 

 on a bridge? On this again our information is but scanty. Sir B. 

 Baker's experiments for the Forth Bridge led him to adopt as his rule 

 double the plane surface exposed to the wind and deduct 50 per cent 

 in the case of tubes. On this point, again, further experiments are 

 needed. 



To turn from engineering to ph} r sics. In metrology as in many 

 other branches of science /difficulties connected with the measurement 

 of temperature are of the first importance. 



I was asked some little time since to state to a very high order of 

 exactness the relation between the yard and the meter. I could not 

 give the number of figures required. The meter is defined at the 

 freezing point of water, the yard at a temperature of 62° F. When a 

 yard and a meter scale are compared they are usually at about the same 

 temperature; the difficulty of the comparison is enormously increased 

 if there be a temperature difference of 30° F. between the two scales. 

 Hence we require to know the temperature coefficients of the two 

 standards. But that of the standard yard is not known; it is doubtful, 

 1 believe, if the composition of the alloy of which it is made is known, 

 and in consequence Mr. Chaney has mentioned the determination of 

 coefficients of expansion as one of the investigations which it is desir- 

 able that the Laboratory should undertake. 



Or, again, take thermometry. The standard scale of temperature is 

 that of the hydrogen thermometer; the scale in practical use in 

 England is the mercury in flint-glass scale of the Kew standard ther- 

 mometers. It is obvious that it is of importance to science that the 

 difference between the scales should be known, and various attempts 

 have been made to compare them, 



