104 AN IMPROVEMENT IN ANEMOMETERS. 



for very strong winds, the Observatory Anemometer has an 

 ordinary scale of 1 inch of paper to the hour, which may be 

 increased to 2 inches, and this is quite enough for all ordinary 

 winds, and more than it is usual to have in Observatories. The 

 new method is one that can at any moment be put into operation 

 by turning an electrical key, and at all other times it is at rest, 

 and costs nothing either in paper or battery force. 



Discussion. 



Professor Threlfall asked if it were possible to calculate wind 

 pressure. 



Mr. Russell : — Not satisfactorily, but a certain rule is 

 generally adopted. There is no question perhaps which is more 

 discussed nowadays as to what is the actual pressure of wind. 

 All the attempts to record the pressure of the wind on any 

 pressure plate have practically failed for several reasons : first, 

 owing to the recoil of the wind that strikes the plate its surface is 

 virtually extended, and gusts at a velocity of 60 or miles an hour 

 or 100 feet in a second strike the plate as if it were struck a blow 

 from a hammer. I have found in an ordinary north-easter, where 

 the wind was not blowing more than 30 miles an hour, that some 

 of the gusts would strike the plate, and make it appear that the 

 velocity was 90 miles an hour, and I was so impressed with the 

 uncertainty that I ceased to trust pressure plates. I think the 

 system called Robinson's Cups is as accurate as any for measuring 

 the velocity, They are subject to some small correction in some 

 cases and not in others, and there is this to be said in their favour : 

 if we measure by such a system as that, we are measuring by an 

 apparatus that can be reproduced hereafter with certainty, and 

 we at any rate get a measure on an uniform system, and if 

 ultimately it is found that the ratio now assumed to exist between 

 the cups and the velocity of the wind be proved incorrect, it 

 nevertheless will be possible to apply the correct ratio to the 

 observations which are now being made, as to the posibility of 

 determining the pressure of the wind by other means. About 

 10 years since, a railway engine was blown right off the rails in 

 America — a heavy freight engine, and the question as to what 

 pressure of wind was required to blow it over was investigated, 

 and it was found that nothing less than 95 lbs. to the square foot 

 would have that effect ; and there are many similar cases in 

 which the wind has actually exerted pressures greatly in excess 

 of what is often assumed to be its maximum . 



Mr. J. S. Mitchell : — Many years ago in Tasmania I was 

 very much interested with a very complete arrangement they had 

 at the Observatory for measuring the strength of the wind and 

 the rainfall, and I took myself to make one. I noticed that the 



