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 physics. 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° Fahr. When a yard and 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° Fahr. 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, I 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 desirable 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 thermometers. 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.
But the results of no two series of observations which have been made agree satisfactorily. The variations arise probably in great measure from the fact that the English glass thermometer as ordinarily made and used is incapable of the accuracy now demanded for scientific investigation. The temporary depression of the freezing point already alluded to in discussing the Jena glass is too large; it may amount to three to four tenths of a degree when the thermometer is raised 100°. Thus the results of any given comparison depend too much on the immediate past history of the thermometer employed, and it is almost hopeless to construct a table accurate, say, to .01 which will give the difference between the Kew standard and the hydrogen scale and so enable the results of former work in which English thermometers were used to be expressed in standard degrees.
VALUES OF CORRECTIONS TO THE ENGLISH GLASS THERMOMETER SCALE TO GIVE TEMPERATURES ON THE GAS THERMOMETER SCALE FOUND BY VARIOUS OBSERVERS.