“Monorail Subway, Capitol To Senate. The Car. Senators Swanson And Townsend As Passengers, 1916”. (Photo by Harris & Ewing Collection). (Please click to enlarge)

A undated handout image provided by the Bochum University of Applied Sciences (Hochschule Bochum) on 04 October 2013 shows the German team Bochum's Cruiser class solar car during a test drive in the Northern Territoty, Australia, 16 September 2013. The Dutch vehicle is competing in the World Solar Challenge, driving from Darwin to Adelaide starting on 06 October 2013. (Photo by Hochschule Bochum/EPA)

Photographers gather to take pictures of pilot Andre Borschberg (C) ahead of the take off of Solar Impulse 2-a solar powered plane- in Nanjing, China, May 31, 2015. The world's largest solar-powered airplane, Solar Impulse 2, took off from eastern China's Nanjing on Sunday to continue its round-the-world voyage. The Swiss-made plane left Nanjing's Lukou International Airport at 2:39 in the early morning, with former fighter pilot Borschberg at the controls alone for the entire 8,200-kilometer flight from Nanjing to Hawaii, the toughest leg of its marathon adventure. (Photo by Reuters/Solar Impulse)

The shadow of a partial eclipse is cast on to the cheek of a student on the roof of the Jana Dlugosza Academy in Czestochowa, Poland March 20, 2015. (Photo by Grzegorz Skowronek/Reuters/Agencja Gazeta)

People watch a total solar eclipse from the Bay Bridge in Palu, Central Sulawesi, Indonesia March 9, 2016 in this photo taken by Antara Foto. The rare astronomical event is being witnessed Wednesday along a narrow path that stretches across 12 provinces encompassing three times zones and about 40 million people. (Photo by Mohamad Hamzah/Reuters/Antara Foto)

A balloon depicting the face of alien movie character E.T. is seen in the sky at the 15th Solar Balloon Festival in Envigado, Colombia December 31, 2015. (Photo by Fredy Builes/Reuters)

A solar-powered plane takes off from Jiangbei International Airport in southwest China's Chongqing Municipality, Tuesday, April 21, 2015. The Solar Impulse 2 departed Chongqing on Tuesday for a 1190-kilometer (642-mile) flight to the city of Nanjing in eastern China, the sixth leg of its around-the-world flight. (Photo by Chinatopix via AP Photo)

A woman squeezes between two public buses in downtown Lima, March 17, 2014. Bogota and two other Latin American capitals – Mexico City, and Lima in Peru – were named as the three capitals with the least safe transport systems for women in the Thomson Reuters Foundation poll of more than 6,550 women and gender and city planning experts. Women in Latin America say they face a wide range of daily threats on public transport, and not enough is done to ensure their safety. (Photo by Enrique Castro-Mendivil/Reuters)