Find a Nth highest or lowest number in a given series of numbers using TSQL

DECLARE @T TABLE(Number FLOAT)
INSERT INTO @T VALUES(1),(2.06),(-36.4),(-4),(5.90),(618.9),(79.8),(8),(9),(18.0),(23.7)
SELECT HighestNumber=MAX(Number), LowestNumber=MIN(Number)
FROM @T

HighestNumber	LowestNumber
618.9	       -36.4