304 PROCEEDINGS OF THE AMERICAN ACADEMY 



XVII. 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF 

 THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



XVI. EXPERIMENTS ON THE FATIGUE OF SMALL SPRUCE 



BEAMS. 



By F. E. Kidder. 



Presented May 10, 1882. 



The following experiments were undertaken with the object of deter- 

 mining if possible what part of the so-called breaking load of a beam 

 would ultimately cause the beam to break, all the conditions being the 

 most favorable. 



Incidental to the experiments, the moduli of rupture and of elas- 

 ticity of small beams of kiln-dried spruce were determined. 



The experiments were made in the Physical Laboratory of the 

 Massachusetts Institute of Technology, the testing-machine used being 

 the same as that described in a paper by the writer presented to the 

 Academy Feb. 9, 1881. 



With this machine the loads are applied by suspending known 

 weights directly from the centre of the beam. The deflections of the 

 beams were measured by means of a micrometer screw, the principle 

 of electrical contact being taken advantage of in reading it. The 

 moduli given have been computed from deflections measured to 

 thousandths of an inch or hundredths of a millimetre. 



As the load was suspended from a bolt resting upon the beam at 

 the centre, it was necessary to measure the deflections one inch from 

 the centre. For the small deflections from which the moduli of elas- 

 ticity were determined, the difference between the measured deflection 

 and the actual deflection is so small that it would not come within the 

 limit to which the deflection was measured. For the deflections given 

 in the tables, the deflections at the centre would be somewhat larger, 

 but the error does not practically affect the results. 



As the room in which these experiments were made is kept very 

 warm and dry, any unseasoned timber would be so aff'ected by the 



