NATURAL PHILOSOPHY. 149 



At Lawrence, with a fall of thirty-four feet, the vibrations are not no- 

 ticed when the depth of water is much less than three feet ; but this 

 seems to be owing to the inequalities on the top of the darn. The vi- 

 brations cease almost entirely when the water exceeds a certain height 

 because the thickness of the sheet becomes too great in comparison with 

 its height, and there being some cohesion between the particles of the 

 liquid, the sheet partakes somewhat of the rigidity of a solid body. In 

 order to produce a strong effect, the thickness of the sheet must not ex- 

 ceed about one-sixth or one-eighth of the height of the fall. At South 

 Natick, with a fall of nine feet, which is somewhat diminished by the 

 back- water at the time of a freshet, the vibrations cease when the depth 

 of water much exceeds ten inches. At Holyoke, with a fall of thirty 

 feet, which is also diminished by the back-water at the time of a fresh- 

 et, the vibrations cease when the depth of water much exceeds five 

 feet. At Lawrence, also, where the fall is a h'ttle greater than at 

 Holyoke, the vibrations cease when the depth of water, on the crest 

 of the dam, much exceeds five feet." 



According to these views, all dams may be built so as to avoid jarring 

 vibrations. 



SCIENTIFIC BALLOON ASCENSIONS. 



Under the auspices of the British Association, the balloon ascensions 

 inaugurated in 1862, for scientific observation and experiment, have 

 been continued during the past year by Mr. Glaisher the well-known 

 British meteorologist, and the former aeronaut, (see Annual Sci. Dis. 

 1863, pp. 137-144.) At the last meeting of the Association, this gentle- 

 man gave the following resume of the facts and deductions arrived at in 

 his recent ascensions : 



On ascending with a cloudy sky, the temperature usually declines till 

 the clouds are reached ; but on breaking through them, there is always 

 an increase of several degrees of temperature ; and after this the de- 

 cline of temperature usually continues, and would do so continuously 

 if there were no disturbing causes in operation. On ascending with a 

 clear sky, we start with a "higher temperature than with a cloudy one 

 as much higher as the loss of heat caused by the clouds ; an approxi- 

 mate measure of which is that sudden increase of temperature in passing 

 from cloud to a clear sky. In no instance have I met with the atmos- 

 phere in a normal state in respect to temperature ; at different eleva- 

 tions even up to four or five miles, warm currents of air have been met 

 with. By warm, I mean 'that their temperature was higher than in the 

 stratum beneath. These warm strata are variable in thickness, from 

 1,000 feet to 10,000 feet, and varying from 1 to 20 in excess. It is 

 necessary, in considering the law of the decrease of temperature, to 

 take into account the state of the sky, and to separate the experiments 

 made in one state from those in the other. The results in the cloudy 

 state do not at all confirm the theory of a decline of 1 of temperature in 

 300 feet. If we now consider the decrease at heights above the cloud 

 plane, the decrease of the temperature of the air, at heights exceeding 

 5,000 feet, the results follow almost in sequence with those found 

 with a partially clear sky, and show that an average change takes 

 place of 1 of temperature in 139 feet near the earth and that for a 

 change of 1 at the height of 30,000 feet, we have to pass through at 

 13* 



