•282 THE PERUVIAN TIDAL WAVE. 



of July 15th, a condensed statement of a new theor3^ in relation to tor- 

 nadoes : 



"The Signal service has given us a new fact in relation to tornadoes 

 which has given rise to a theory that may lead to another link in the chain 

 of investigation res^Decting the law of these terrible visitants. 



"Its nomenclature is borrowed from the railway, and termed the thermal 

 gradient. Take for example the late tornado at Pensaukee, Wisconsin. At 

 the time of its occurrence a steejo thermal gradient prevailed between the 

 Mississij)pi and Lake Michigan — in length about one hundred and fifty miles. 

 At La Crosse the thermometer stood at 95° and at the Pensaukee end it was 

 65°, a difference of 30°. This condition, it is claimed, with the difference 

 in the atmospheric density and humidity, furnishes the conditions neces- 

 sary to develop the tornado. 



"The character of the movement is a violent rupture of the superin- 

 cumbent strata of dense cold air by the highly heated surface stratum. In 

 the effort of the latter to expand and obtain an outlet it acts like steam 

 when confined in a boiler. If this is correct, the greater the difference in 

 temperature between the ^Joints on this thermal gradient, the more violent 

 will be the atmospheric explosion. 



"The gradient in the case of the Pensaukee tornado, was a descent in 

 the temperature of 1° for every five miles. A tornado then under this the- 

 ory was inevitable. Whether the theory is a correct one, is open to discus- 

 sion, but the fact is not, and therefore valuable, because it is from the accu- 

 mulation of facts that the law will ultimately be established." 



THE PERUVIAN TIDAL WAVE. 



(COMPILED BY THE EDITOR.) 



On the 10th of May, 1877, a great tidal wave brought destruction to 

 many thriving towns along the coast of South America, and especially along 

 that of Peru. The appearance and effect of this wave is Avell and fully de- 

 scribed in the extracts given below from various local papers, but there 

 seems to be some disagreement among observers and theorists with regard 

 to the origin or starting point of the wave itself. The journalists of San 

 Prancisco seem to attribute it to an earthquake occurring on the evening of 

 the 9th of May, resulting from an eruiDtion of the volcano Ilaga, situated 

 on the southern borders of Peru and Bolivia, while a writer in the Christian 

 Union, from the Hawaiian Islands, connects it directly with an eruption 

 within the crater Kilauea, and a subterranean upheaving of the water in 

 Kealakeakua Bay. On both coasts at about the'same time violent shocks of 

 earthquake were felt, great disturbances of the ocean were observed, and 

 immediately following came immense waves, varying at points along the 

 South American coast and that of Hawaii from five to sixtj^-five feet in 

 height, which swept away or seriously damaged many towns and destroyed 

 thousands of lives. 



